I'm six years into a fort on a 6x6 map that has a couple of extreme cliffs, a heavily forested river valley, and a cave that had some Antmen until they swarmed and died. I have a max pop of 80 (though my current population is ~95), have the Orc mod, and modded in a custom megabeast with [FLIER] and [AMBUSHPREDATOR].
I got sieged by Orcs my second year, and afterwards fought off two or three goblin ambushes. it was great, constant action! Two or three years ago, though, the action simply stopped. The best I get now is a kobold thief. Where have all the orcs and goblins gone? They found me just fine before, and it's not like our small skirmishes should have decimated their numbers.
I considered the possibility that a goblin ambush or an [AMBUSHPREDATOR] megabeast were lurking somewhere on the map, so I sent a squad to do a couple laps, but they found nothing.
Any ideas? The best I can think of right now is,
1) Goblin ambush or megabeast sitting somewhere and bad luck just keeps me from finding it.
2) An [AMBUSHPREDATOR] megabeast is FLYING somewhere on the map, and won't come down and be found. (But wouldn't I have gotten a notice when it appeared on the map border, at least?)
3) Part of a human caravan has been bugged and stuck in my depot for years, even though other caravans have used it normally since. Maybe it's somehow related to the disappearing hostiles?
4) Some world event coincidentally wiped out both the orcs and the goblins? Is this even possible in the mere 6 years my fort has been around?