Short answer: no.
Long answer:
The best knowledge we have about the inner workings of the raid system today is based on leaked files from several years ago. However, it has probably not changed much since then. From that info, we know raids have specific spawn points with ranges, and that each monster/boss can spawn in certain pre-defined amounts. A message announcing the raid is totally optional.
Consider this example (not real data)
Creature: Dharalion
Amount: [1,1]
Spawn Point: (33038,32177,9)
Range: 2
Message: <blank>
Creature: Elf Arcanist
Amount: [1,3]
Spawn Point: (33038,32177,9)
Range: 5
Message: <blank>
This means that when this raid is picked by the server, it will spawn Dharalion no more than 2 tiles away from that coordinate, and either 1, 2 or 3 Elf Arcanist somewhere 5 tiles around there. It will not send any messages (so it is an unannounced raid).
With these parameters, we can describe pretty much all raids (announced or not). A "surprise" boss would have to be described with its amount as [0,1], meaning 0 or 1 of them can be created with a raid. And given how old certain announced raids are, we can be quite sure that no unexpected bosses are coming out of them - we have collectively killed them too many times and we would have seen it by now.
But what if the spawn chances are very low when we have [0,1]? Well, we had recent confirmation from CIP that some bosses do indeed have a chance of not spawning with their raids (while some of its minions still show up) - however, we also see these bosses showing up just as often, so it seems like the chance is just 50/50.