Ultegra rear derailleur mystery piece

Good afternoon!

This might be a long shot, but i've seen some amazing replies in here it's worth a try.

Yesterday when putting back the back wheel, after replacing the inner tube, i bumped a small (~1cm) piece off of the rear derailleur. I've been looking and fidgetting for a long while bit can't figure out where this came from. Looks like a little protection cap. Everything still functions just fine.

Derailleur is Ultegra Di2 (R8150-DGS).

If anyone recognizes this bit and knows where to put it, it wouls be greatly appreciated! Thanks!