At work and trying to remember... do mobs hang around after they kill you ? That's what old eq1 had, and a random time period that they lingered over the fallen. Sounds like the whole things an issue with the way mobs path... i've pulled before only to feign, and noticed roamers run back to where they would have ended up if they'd continued along without my having disturbed them. Like somehow their still tracking the path they would have taken. Sorta seemed like their pathing was too pre-ordained. Maybe they learned it in MOB 101 : Be here or be square!
I believe this is the current idea of the new mob interations with FD.
It will likely end up almost exactly as EQ FD other than it will contiinue to be a single check for FD instead of each mob. So 1 check but mobs return to their spots after FD at varrying rates.