I recently purchased a used XJR engine from a 2004. Upon inspection of the camshaft housings I found aluminum plugs that have been pushed out by some internal engine force. The plugs are supposed to be blocking holes in the centers of each head. They are aluminum and cylindrical.

I'm holding the plugs here. One is from each head.

Look in the the center and you can see the hole. Below the hole you can see the plug resting at an angle.
I'm assuming it's caused by oil. Please see the pictures.

 

The plugs are from the center of the head as seen in this image.