Resident dragons are different from feature creatures -- they are more like goblins living in their own tower. The game doesn't currently distinguish between new and old traps -- it just assumes that resident creatures know as much about the site as you do. That could be improved but would take a bit of twiddling.
I take it this means that this issue would also apply to (say) humans in a town we settle in, or elves or whatever?
Hrm, perhaps it should wait until there's some more concrete concept of the dwarves' "relationship" with whoever else already lives at the site where they're settling... I can see dwarves, say, setting up some sort of peaceful coexistence with a human town and building a fortress underneath it; in that case, the traps probably shouldn't hurt humans (the dwarves tell the humans about them, or whatever.) On the other hand, if your dwarves are essentially invaders sapping into a goblin tower or dragon cave to kill whoever lived there before and take their stuff, traps should probably work just fine...
Of course, it also doesn't make sense that the dwarves could, say, walk into the human's houses while friendly, build a million traps, then suddenly break off the treaty and have the humans magically forget where every single one of those traps was. Tracking all that could be a bit harder... although I suppose it really just needs to check who you're allied with when the trap is first built, and set the trap 'known' to anyone on those sides... And then there'd be the option of, say, telling the humans about the outdoor traps that might concern them directly, but "neglecting" to mention the traps around your treasure horde or the ones deeper inside than the humans have any right to be in the first place, just in case your relationship with them goes sour. And you'd probably only tell traders about the traps that are in places where the traders are "supposed" to go... but you'd kinda have to tell them about those, so if you end up at war with their civilization those traps probably won't be very useful. All this might be a bit too much to worry about...
Dunno if that's the direction things are going in, though. I seem to recall that way back when the first version that allowed it was released, it wasn't certain whether or not you would still be able to settle over other civilization's towns and towers once all is said and done. (Forcing people to use an army and a 'proper' invasion if they want to invade, I guess.)