I have a fort where things tend to appear in strange places.
I started noticing this in the fall of the second year, when I noticed some wild camels running around inside my wall, with no traps triggered, and caught a wave of migrants popping out of a spot in the sand outside.
Throughout the history of the fort there have been a series of camel invasions, where wild camels would show up, climb to the top of the walls, and proceed to relentlessly terrorize the masons (I'm working on building a huge ziggurat).
I was willing to tolerate this, especially when I thought I had narrowed down the mystical camel-materialization nexus to an area in the north of the fort and started digging a ditch around it. Then a goblin ambush spawned in that area, and killed the miner digging the ditch, as well as a few other important dwarves before being subdued.
I finished the ditch, thought I had things under control, and soldiered on. (Interestingly, I've received no more camel invasions since digging the ditch.)
Now I've received a wave of migrants who are appearing at a point in the southwest of the fort, and I've saved while they're in the middle of spawning.
The only thing I can think of that might be related is that I embarked on a particularly wide (east/west-wise) site in order to encompass a stream
and a magma vent, and the observed spawning locations are consistent with the possibility that they're all along some invisible line, perhaps a biome boundary, or what would be the edge of the site if it was a square. I've observed things -- like caravans -- appearing from the north and south, but never from the east or west. (I haven't had a caravan spawn inside the fort yet, but that would be fun...)
Save is here:
http://dffd.wimbli.com/file.php?id=627