I've always been curious about those kind of doors too, so your question has sent me on a little quest through various Tibian cities for the last day or two ;) I wanted to double check the locations I remembered so far, and possibly discover some new ones.
Some of those doors with red glowing dorknoobs belong to the well-known quests, but there are at least three of them which I know nothing (or not much) about; or I'm simply unsure if I connect them to the right quests. But I think that listing them all can lead to some possible new conclusions, so here are the results of my findings, so far:
1) Thais has no less than 5 (!) of those kind of doors:
- the door above the shop with npcs Gamon, Topsy and Turvy, as others have mentioned before, but also:
- the door under the MacRonalds' family farm, near npc Sherry MacRonald (is it where you get the reward from Kissing a Pig quest? I've never finished it yet)
- the one in the Thais sewers (!) under Xodet's magic shop - probably not that many people know how to access it because it's part of the "noobish" The Scatterbrained Sorcerer quest with npc Eclesius, when we have to find "Xodet's first wand"
- the one near npc Gamel, near the place where we get our rewards after finishing the Thieves Guild Quest
- and finally, the door high up in the Thais castle, in the small room adjacent to the Royal Archieves room (I think it's a part of the Secret Service quest for one of the 3 secret agencies, probably not the Thaian one)
Now, about the most mysterious door over the furniture/magic shop - I agree that it's worth talking with the nps there. When we ask Topsy about her "boss" she tells us: "I had one once. He should have bought better armor. Actually - he's upstairs". Now let's analise it just a little bit, shall we? Topsy USED TO have a boss, (but she doesn't have one anymore). He SHOULD HAVE bought a better armor (but he didn't, and something bad happened to him, making him unable to work anymore). And yet... he is still upstairs, exactly where we cannot go, hidden behind a DOUBLE (!) red glowing door... I think that based on this curious answer we may safely assume, that Topsy's boss is DEAD, and that he still inhabits his old room as either UNDEAD, or a GHOST. That assumption gets even stronger when we compare it to another, seemingly irrelevant answer given by Topsy when we ask her about the DUNGEON. This is what she says: "Dungeons - dank, cold places if you ask me. They lead to rusty armour, severe colds and death... On the other hand you use a lot of runes there... So just think about the treasures you'll surely find there". - Isn't it a bit weird, that her first reaction is reminiscing about something personal from her own past, clearly related to someone she knew, who had DIED due to severe cold, caused by a rusty ARMOUR? And then, there is also this little curious line from Sherry MacRonald, who, when we ask her about the MILL gives us this answer: "The miller is a lazy fellow and affraid of his own mill, because he thinks it is spooked". And then let us add all those pieces of information together and let us think about the fact that the mill in Thais is COINCIDENTALY only one house away from Topsy's shop... Personally, I am convinced that the late unfortunate Topsy's boss had met his sad end in some dank, cold dungeon, then CAME BACK to live (or rather to UNDIE) in his old room upstairs and also moonlights at nights as a ghost in a nearby mill... Perhaps he belonged to the family of the miller? Maybe he spent his happy childhood there and tries to recreate and reenact those memories now, not knowing that he is in fact, dead? Or perhaps he just HATES the current miller and deliberately WANTS to "spook him out"? ... But, enough about the mysteries of Thais, lest us come back to the glowing red dorknoobs theme... So the next city which I checked is:
2) Carlin, which has:
- red glowing door under Queen Eloise's castle (I think you access one kind of cultists there in the Cults of Tibia quest - I've never done it yet myself)
- and another door under the Carlin Theatre - I'm not sure if that one is already known in some quest or not... is it where we get the funny book containing a sort of humorous theatre play, which we then have to recite to npc Percybald, in order to get the "Amateur Actor" achievement? Which quest was that again? It's been a while since I've done it...
3) Ab'Dendriel - now that city has another SUPER INTERESTING, mysterious door, which is not connected to any quest, as far as I know. It's located over a small cave where some elves seem to be conducting some kind of nasty rituals involving human sacrifice... or they are perfectly innocent elves minding their own business, but they were disturbed by some nasty nosy hostile humans, who were then utterly defeated. Either way... There are 3 human bodies/skeletons there. The cave is located on the west side of the city, north from the labirynth with the Sacred Giant Tree - it is not immediately visible, we have to cut some high grass to get there, and then rope ourselves or levitate one level up. The door with the glowing red knob guards a little room where we can see one of the dead human bodies over what seems to be a "closed" hole in the ground (?) - if we go down we can see a brown bag immediately under it, as if the imprisoned human tried to escape or at least throw down his bag, before he died. The bag was empty when I checked it, but it 's possible that someone else was there before me that day and took whatever was there. Or maybe it's one of those rare respawns which only renews itself after some major update (?) Anyway, I don't expect any great treasure in the bag, but it would be great to find some clue in it, as to the identity of the man behind the red glowing door above...
4) Venore - I've only found one red glowing door there, in the old and messy shipyard (I think it's for another mission of one of the 3 possible Secret Service quest lines, when we have to steal a plan of some Venorean ship) This shipyard is interesting in itself... but that's a whole another story.
5) Port Hope - also only one red glowing door there, in a little wooden house, east from the depot, where some wolves are entrapped. And again, I think that this is also one of the Secret Service quest missions. (I think we can access another half of this little house by the underground passage leading from the Smugglers caves, if I'm not enrirely mistaken).
6) Darashia - this is another mystery in my book. There are red glowing doors in the seemingly irrelevant, empty (uninhabited/unocupied) house, west from the ship. Is this connected with some super hard quest for high leveled players, which I know nothing about, like the Ferumbras Ascension, or something? Please satisfy my curiosity, if it's something obvious for people more experienced in the game than I am ;)
7) Ankrahmun - now this is an odd finding for me... Not because I found any red glowing doors there, but because I DIDN'T find them. Allow me to explain - I was absolutely sure that there are exactly those kind of doors in the Serpentine Tower, in the room with some magical artefacts, on the opposite wall to the 3 monsters entrapped there. I went there to confirm that my memory is right before wrighting this huge post and... the door were locked as always, but they weren't glowing. So my question for the more experienced lorekeepers is - did I just imagine the red glowing doorknob there and my memory is entirely wrong in that matter, or did Cipsoft possibly change something there? I was really taken aback by NOT having found the doorknob glowing in red...
Anyway, I haven't check any other cities YET and you mentioned seeing those kind of doors in Kazordoon and Beregar, so I didn't check there either. It is also entirely possible that I have simply forgotten about some of these doors hidden in one of those cities which I have already visited in my little "red door quest" (for example, I didn't check entire Hellgate in Ab'Dendriel yet). And finally, it's also possible that I simply don't even have access to some high-level places and quest areas yet, and therefore I don't even know where to look and what to look for... If I remember about any other red doors or discover any new information, I will edit my post, of course, or write an entirely new one... And if anybody knows about any other red glowing doors of this kind, please DO share :)
-------------------------
But before I go - I had another little discovery today, which I noticed completely by pure accident, just because I was already on this whole "doors with red glowing doorknobs quest". And that is...
8) Dawnport - there are 4 red glowing doors even there (!) Those are the doors we have to enter right after we have chosen our vocation. Which means that 3 of them "seem to be sealed against unwanted intruders", and only one of them open for each of our new characters. But there are a few curiosities which I have noticed there:
- 3 of those doors are indeed red, but the 4th one is... BLUE and ornamented, with a BLUE doorknob which doesn't glow at all. The blue door (for the oh-so-special sorcerers) works exactly as the other 3 howerer, with exactly the same message when we "look" at it, so that's why I count it as the 4th (which might mean that those kind of door do not have to always look the same).
- there is no animation of opening them when we enter, but rather the blue light of a teleport, taking us directly into the room when we use them. Similarly, there is the same blue teleport light when we leave the room, even though there are no doors there at all, just an "arch" leading out. (Which means that those kind of glowing doors behave like teleports, not like normal doors opened with a key or with the "use" function).
- and perhaps the most important of my conclusions is this:
ALMOST ALL of those above mentioned "red glowing doors" seems to open for SOMEBODY, providing that they fulfill SOME NECESSARY CONDITIONS (like being on the right quest, having the right level, or the right vocation etc). So that makes me think, that EVEN the most mysterious doors in Thais or Ab'Dendriel, or even Darashia (if those last ones are also still unknown) MUST and WILL open for us someday, IF we fulfill THEIR requirements to enter... I wonder who will be the first to achieve such a feat...
And that's all I have to say about this topic... for now!
But something tells me that I will surely return here soon, hopefully more then once...
Good luck to all the Tibian bookworms, lorekeepers, discoverers and questers out there! :)