Cleaning my engine bay and found this cap - anyone know what it's for?

[ZR1crusader]

I'm sorry if this is a stupid question...it probably is...but here goes:

I've spent hours and hours now cleaning (detailing) my engine bay. In doing so, I found this cap lodged in down in the engine bay. It wasn't this clean when I found it - it was caked with grime and dirt. I have a pic with a red circle highlighting where I found it and another picture of the engine a little further away to set better context.

Anyone know if this is actually something for the engine that I should put back where it goes, or is it just a cap accidentally dropped and left there by the previous owner?