I've had several undead sieges which consisted of one or two living non necromancers (often goblins) plus a number of undead. I've thought the living ones were minions of the necros, who didn't bother with such a minor siege (alternatively were skulking and never revealed themselves).
Invaders unable to find the fortress has been a plague of mine for quite some time. Sadly, it's not unusual for invaders to drift aimlessly along the edge of the map, back and forth. My worst case was a force that drifted for a year before giving up. The morons have at times even entered my wide open trade depot entrances, only to back out again.
I suspect (no proof) invaders have a number of pathing issues:
- Too far away from the fortress might be a factor.
- At least for undead, one of them finding the fortress does not result it communicates the location to the rest of them. I've recently had a drifting force where 4 undead stumbled upon the fortress and got caught by my cages while chasing local wildlife. The rest of the force eventually left as the new season arrived.
- Morons entering trade depot entrances and backing out might have been thrown off by the fact that the trade depot have been located lower than the meeting area/mess hall, and in order to reach the meeting area they'd have to pass under it and then go up.
- Once a force (or I suspect its leader in the case of undead) has detected the main entrance, using the trade one suddenly has no longer been an issue, so I've then been able to have them running back and forth as I close/open the drawbridges.
- Enemies appearing behind a volcano have had extreme difficulties pathing to the fortress located just on the other side of it (but a lot lower). I suspect first pathing up and then down is difficult, especially if the main entrance is facing the other way.
- Because of enemy drifting, I now embark only on perfectly flat terrain, and make sure the trade depot and the meeting hall is on the same level.