What part of my engine is this?
 

What part of my engine (Volvo S80 Petrol Non Turbo (B5244S02) is this? Found it on the garage floor under the engine.
It was very much covered with old oil when I found it. My best bet is that it might come from the (very inaccessible) crankshaft ventilation system.

Never seen one of them before. Looks like a cap off of an lawn mower. Maybe you picked it up off the road and it finally fell off when you stopped in your garage.