Well, to be true, I *do* receive a thief warning. The game stops, the view jumps to the thief, and I watch how he (it?) is dealt with by the watch while the other dwarves flee. That ones's dead, no doubt about it.
A minute later, I see blood in the corridor, two triggered traps, and two goblin corpses. The one that was slain in the open, and another *first* trap.
When I look the next time, a third trap has been triggered, and there's a grand total of three dead goblins.
I was too occupied with other stuff to watch my traps as closely when the next snatchers struck, but it appears that not all are detected in the open. Detection always takes place in fornt of the cave (and is lethat to the goblin in question), but time and again I find blood and triggered traps quite a bit in the cave. Maybe the guardsmen pursuing and civilians fleing creates a sufficiently large window of opportunity for subsequent goblins to enter unseen.
However, there's also a constant traffic to the outdoors, these dwarfs are blissfully ignorant of the scene just round the corner. They should encounter the goblins on their way in; the only explanation I can come up with is that movent appears to happen in a clockwise fashion: the dwarfs going out hug the lower (southern) wall, while on their way in they keep to the upper wall. As do the goblins, or at any rate they always trigger the upper traps. Apparently a corridor six tiles wide offers enough space for the goblins to go unnoticed.