Loose brake cable hook bushing inside the brake/shifter hood
The little piece is the pivot bushing for the lever shaft. Its' job is to prevent a metal on metal contact point. You can reposition it by loosening the cable clamp at the brake caliper, gently slide the little barrel out far enough to install the bushing. By pulling the cable back every thing should go back in place.

It can be helpful to have a pair of needle nose pliers nearby, as you might need to pull the cable a bit to make the barrel move far enough. Also, if possible, move the shift/brake lever inwards towards the middle of the bike before closing the brakes to open up the hood as much as possible.