I generated a corpse in plane of mischief zone that had the standard 3 hours of experience res time, with 6 days before completely expiring. I did not log out of the game and returned to the zone approximately 3 hours after dying and the corpse was missing. This specific thing has happened more than once. Is there a mechanic I am missing?
That's not how it works any more. Corpse has 6 days only if you are offline. 3 hours of online time, or time at character select, and then it disappears. Since there is no need to return for gear any more, there is no reason for a corpse to persist past the rez timer expires. You were probably a bit after 3 hours getting back.
