I am really reluctant to report this as a bug or otherwise but felt I really need to at least make you guys at SMP aware of what I saw and you can decide if you want to try to duplicate it if you like, though I don't see any possible way of duplicating it myself.
The situation is simple.
I saw 3 enemies come up to level 2 on my map, they apparently were following one of the Heroes/Adventurers out of the dungeon.
They fought with one of my townies and killed her, then walked from the hallway where they killed the townie into my main underground room, they moved no more than 4 cells and then promptly disappeared. There were no townies around to attack them and no Heroes around either. They simply vanished into thin air. I watched them while they were fighting the townie and all 3 of their health level was good or better so it's not like they took sustained damage and then died 3 steps later.
Anyway, in short, TL;DR as they say, the enemies simply vanished without cause or reason. And NO, I've never seen this happen before.
I do have a save file if you should need it for some reason, although it was saved right after this event occured so I doubt it will be of any help to you, but I'll hang on to it just in case.
Yes, my game is slightly modded, to include the tree/bush mod, as well as simple modding by myself like increasing the amount of items containers will hold, as well as being able to place things like farms underground and mining shafts on both silver and gold.