The valve covers have a collar on them......
I have taken motors apart and had stuff "show up" that I don't remember. Sometimes, it was stuck in the gunk that was covering the motor. Dropped there during previous work. If you can't find where it goes, I'm sure it will be fine. I wouldn't use any sleep over it. It's probably a spacer, or a sleeve. It sounds like you double checked all the dowels, so, look it over once more and see if you notice anything. Usually internal engine parts are not coated, so I'm pretty sure it's nothing that will cause a failure. Good luck!!
