2001 S80 T6 was burning oil from engine compartment. Lifted the hood and discovered one of the two metal caps on the engine head had come almost completely off and was allowing the oil to escape and burn off on the engine. Are these caps pressure relief caps and what could cause only one of two (the other is completely seated and flush with the head) to come off? Are they sealed with a gasket?

 

Most likely the crank case system is clogged.
They are pressed in there like freeze plugs are in the block.
The bad part is the proper way to install them is to remove the cam cover and tap it in from the back side.