This is actually a very interesting question...
Regarding the raids without announces, you need to always be checking in-game, in the locations where the invasions occur. This applies to both the Femor Hills raid you mentioned, as well as Nemesis boss raids, Bestiary raids, and other raids that occur regularly without announcements.
There are several raids without announcements, I will list only the raids without announcements that I consider most interesting to check:
Orc Invasion in Femor Hills. (Because there's a chance of dropping items from the amazon set on the orc warlords of this raid.)
Orc inside the Orc Fortress. (It's an orc that spawns in a specific location that drops the "old and used backpack".)
Midnight Panther in Port Hope (Bestiary and Mount)
Undead Cavebears in Lich Hell (Bestiary and Mount)
Crustacea Gigantica in Treasure Island or Calassa (Bestiary and Mount)
Yeti in Folda (Bestiary and rare loot)
In addition to the vast majority of Nemesis bosses that make rare invasions and you must check the location where they spawn to be lucky enough to encounter them.
As for raids with announcements, in the past, you had to pay attention to the game messages, but nowadays, thanks to the Measuring Tibia Quest and the Tibia Observer app, we can see all raids (that have an announcement) directly from our phone and up to 1 hour in advance (for the area) and 15 minutes (for the sub-area), in addition to a notification as soon as the raid begins.
There are some raids with announcements that are interesting to keep an eye on, in case you want to complete some Bestiary and Boss entries and also to get some rare items, I will list them below:
Raids in Carlin:
Troll Raid North-West of Carlin (Troll Guard Bestiary)
First message "Trolls trolling north-west of Carlin!"
Wasp Raid North of Carlin (Nemesis Boss Willi Wasp)
First message "Some wasps have been found north of Carlin. There is some loud buzzing in the air," the boss appears 5 minutes after the first message.
Raids in Darashia:
Lions west of Darashia (Nemesis Boss Zomba)
The first message is "Hungry lions scout the western Darashian desert. Travellers beware!" and the boss spawns after 5 minutes.
The Pale Count in Drefia (Nemesis Boss The Pale Count + rare loot + rare Bestiary)
The first message is "The Pale Count has risen from his crypt deep under Drefia. Blood will flow." and there is 2 very rare bestiaries to do there, Vicious Manbat and Nightfiend
Raids in Edron:
Orshabaal Raid near Edron (Nemesis Boss Orshabaal + rare loot)
The first message is "Orshabaal's minions are working on his return to the World. LEAVE Edron at once, mortals," and the boss spawns 6:40 minutes after the first message.
Morshabaal Raid near Edron (Nemesis Boss Morshabaal + rare loot)
This boss usually appears on server save with the following message: "Beware mortals! Orshabaal's brother, the infamous Morshabaal, is out for revenge and will spread fear for his family once again!" he remains available until he is killed or until the next server save.
Raids in Liberty Bay:
Pirate Boss Raid on Liberty Bay (Pirate boss that drops items to make the addon):
In this invasion, you can find the bosses Ron the Ripper, Lethal Lissy, Brutus Bloodbeard, and Deadeye Devious.
The first message is "Pirates are launching a surprise attack on Liberty Bay! Take care, they seem to be everywhere," and the bosses appear 10 minutes after the message.
Mogaroth Raid in Goroma (Nemesis boss Morgaroth + rare loot):
If you see a raid on the island in the Goroma subarea, you can descend the volcano as the only raid that occurs there is Morgaroth's.
The first message is "The ancient volcano on Goroma slowly becomes active once again," and the boss appears exactly 12 minutes after the first message.
Ferumbras Raid on Kharos (Nemesis boss Ferumbras + rare loot):
If the subarea is Kharos, you can prepare for the arrival of the famous Ferumbras, with his expensive Ferumbras Hat.
The first message is "The seals on Ferumbras' old citadel are glowing. Prepare for HIS return, mortals," and the boss appears 20 minutes after the first message.
Raids in Svargrond:
Ghazbaran Raid in the Formorgar Mines (Nemesis boss Ghazbaran + rare loot)
The first message is "An ancient evil is awakening in the mines beneath Hrodmir," and the boss appears 20 minutes after the first message.
Raids in Zao:
Any Raid in Muggy Plains (Whenever there is a notification in Zao, in the Muggy Plains subarea, you should keep an eye out as there are only two invasions there):
Draptor + Boss Grand Mother Foulscale
The first message is "The dragons of the Dragonblaze Mountains have descended to Zao to protect the lizardkin!," Draptors will appear 15, 20, and 25 minutes after the message, and 25 minutes after the message, the Grand Mother Foulscale, a nemesis boss, will also appear.
Boss The Blightfather
The first message is "Like a swarm of locusts the dreaded lancer beetles are pouring over the fertile parts of Zao," and the boss appears 30 minutes after the message.
Raid in Zzaion, with the nemesis bosses Zulazza the Corruptor, Cublarc the Plunderer, and Chizzoron the Distorter.
The first message is "A massive orc force is gathering at the gates of Zzaion." Cublarc appears 20 minutes after the first message, and Zulazza and Chizzoron appear 30 minutes after the first message.
Orcs in Zao Steppe (Boss Cublarc the Plunderer)
The first message is "An orcish horde, ready for murder and plunder, is amassing to begin its travel through the steppes of Zao. Beware!" The boss appears in 5 (on the left), 18 (in the middle), and 30 minutes (on the right).
To get a preview of the raid areas, you can discover them through the Measuring Tibia Quest.
This quest works as follows: you mark a subarea of the map to discover and search for 7 points on the map (which can appear throughout its extent). By finding the 7 points, you complete the subarea, and by completing the subarea, you complete a part of the total area that you are discovering.
After unlocking 30% of the total area, the NPC locations within that area will be displayed on the cyclopedia map;
After unlocking 70% of the total area, you will be able to see the passages between subareas on other floors and teleports with the symbol of a staircase on the cyclopedia map;
After unlocking 100% of the total area, it is possible to see the creatures that are in all subareas. The creatures that you have not yet unlocked in the Bestiary will only appear as silhouettes. Additionally, you will receive a notification 1 hour before any invasion that will occur in the area. Finally, you will also receive the Achievement related to that area.
And using Tibia Observer App, you will receive notifications of raids on your server in real time.