Rear cam plug blew out from excessive crankcase pressure. That pressure is usually caused by a blocked crankcase ventilation system. This system entails the flame trap and oil separator box which you must keep open for the engine to "breathe" using vacuum created by piston rotation in the intake strokes (that's the principle, more or less)

So, presssure (where there shold be vacuum) has blown out the rear cam plug. Look, feel behing the cam cover at the back of the engine, then down some.

You can read about all that here:
http://www.brickboard.com/FAQ/700-900/EngineSealsBeltsVent.htm#Crankcase_Ventilation_%20A_Treatise

Don Foster occasionally posts his photo of the rear cam seal, and his home-made plate to secure it so it won't blow out, as yours did.

--
"Can't understand why people abort Volvos, either"