I have the same problem on one of my ML parcels...a mystery prim appears, I can never find it, then it goes away. I suspect it's up in the air somewhere and that's why I'm having trouble locating it.
The object, when it shows up in the Object List, always has the same "Most Recent" date, which was a couple of months ago. Autoreturn does not always work on it for some reason, either. It's a real mystery!