Nickel sized piece in oil pan
 

C5 S6 4.2
Dropped the lower oil pan to replace the gasket and found a nickel sized piece in the oil pan:
(The piece did not come out with the oil during the oil change. I only found it while cleaning out the oil pan)

https://goo.gl/photos/8HDikrVG6iYFjPCu8

https://goo.gl/photos/yKqJ5NQZsgSgFahYA

The engine runs just as smoothly with no CELs or anything indicating impending doom.

At first I thought it was maybe the underside of a cam lifter but various people have said it would have been unlikely for it to have traveled all the way down to the lower pan without mishap.
My current thought is that the circular score on one side is likely from a bolt coming up against it. If this was the scenario then a bolt might have dislodged this piece but from what?
- Oil pump?

Any thoughts would be very welcome.

With out question hat is a chunk of aluminum that busted out because someone put a long bolt in a shallow hole. When they felt resistence they just kept cranking on it until it broke the block.

It happened when someone was doing a repair, could be recent, could be old.. but long story short there is now a hole in your block somewhere.