Quote
This is a plug on the outside of the head, on the back side.


That should be the cam plug. It is a metal plug with rubber coating. It is held in place by the bearing cap that holds down the rocker shaft. I think if it was not installed on
re-assembly there would have been huge oil leak right away. I guess if it was installed on an angle or not in all the way it could have fallen out, but it would be hard to install it incorrectly. You will need to remove the valve cover, rocker shaft to replace it. I would talk to the guys who did the work and claim it under warranty. Very strange.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles