Sigh.... Help. Can you identify this bolt

[Regimof]

As some of you know, I recently sent the yellow car in for some surgery. I got it back and everything seemed good (though slightly slower....could be in my head)....

Fast forward to today... Out for a spirited ride and I hear a noise like I ran something over and I feel it rattle underneath. I look back and I see something bouncing in the rear view mirror. I immediately stop... panicking...

Anyway. I found this bolt and a spacer. Both really warm but not too hot to touch.

Does anyone recognize this thing?

Edit: I realize asking someone to identify a bolt in an engine bay littered with them is far fetched. It's just the mechanic was asked and he denies it being anything he touched