Something is probably stealthed. I recommend flooding the entire map with magma, just to be sure.
Failing that, wander around with your military to try and spook it out.
Wouldn't exterminate reveal stealthed creatures? I don't think there are stealthed creatures.
This has happened to me too. I think it was a result of forcing multiple sieges of different types at once. Anyway, I was never able to fix it.
I haven't forced sieges, this one was natural.
Well, if you're on grasslands, you can always use the Liquids command to start grass fires (spawn 1 magma, enjoy inferno) and burn everything on the map to death.
It's kind of a drastic step, but it's always worked for me.
I'm in the desert. I might do that if all else fails, but I'd have to bring in my sheep and also I'd lose some of the ~700 cage traps that I have on the surface.
Possibly an ambush floating, or something stupid. SIEGE offset isn't known. Did you cage any of the invaders? That might be preventing the SIEGE tag from clearing.
This sounds like the most likely. I did cage many of the invaders, but I killed them all in an attempt to clear the tag. Usually, though, sieges are ended when all participants are caged.
I think it may be that the humans started retreating before all of them had entered the map, leaving some in limbo on the edge of the map. I'm going to copy the save, abandon, and check legends.