"Freeze Plug" in Engine Pops Out on 2001 Volvo S80

Drove home and car was smoking. Back freeze plug (as mechanic called it) had popped out and oil leaked on hot engine. Dry dipstick. Replaced oil, put plug back in best I could and drove to dealer. They ran a crank case pressure check ($250) and it was normal/could not find source of problem. No error message, warning light, and BTW where did all the oil go? They dunno, advised to watch and see. Engine is still hot several hours after driving 11 mile commute daily.
Two weeks later, engine is still hot several hours after driving, temp gauge normal, no obvious oil leak, but replacement freeze plug is no longer flush; starting to lift on one side, and drop on other. Guess its back to dealer. Any advice? I dunno is really not a comforting answer from the dealer.

Asked by for the 2001 Volvo S80
Freeze plugs are for coolant, it's an access or oil port plug.
$ 250 for a crank case pressure test? Darn, I work for peanut! You could get a digital manometer for $ 100 and a Volvo oil cap for 20 and make your own pressure tester which will show you the results in a sec.
Great answer from Dan.
The S80s are notorious to have bad breather (PCV) box, clogged breather hoses and oil port at the side of the engine block. They have a banjo bolt under the intake manifold, which clogs - so there are so many PCV problems, which could be also intermittent. I'd also look into this.
Did you ever get the breather control replaced?

Not yet. I was assured by the dealer that their technician did check the PVC box/ breather, all hoses, etc and could not find any issues with that system either.
How many miles are in the T6?
If you remove the oil filler cap and check the bottom of it, what do you see? is it clean, do you see the shiny metal, or there is a bunch of sludge, carbon deposit on it?
The diaphragm inside the breather valve can fail / stuck intermittently and might pass some test in the shop, while it fails when you drive it hard.
I wish I'd see a picture of the oil filler cap.

1 more answer , 1 more comment
If it is an oil passage freeze plug, I recommend doing a mechanical oil pressure test, to make sure that the oil pump pressure release valve is not sticking, which could create excessive oil pressure and force out the plug(s).
Excellent; this is next on my list to have checked. Thank you.