Part Identification Challenge

[Thainfamous20v]

I brought my new bike in for it's maintenance and when taking it out of the trunk, I noticed this piece. It could be nothing important, but maybe someone more familiar with bikes and bike parts could help. Any advice would help.

It's about a half in. wide and 1/3" tall, glossy black.

The 2nd picture has small teeth on the inside.

I have a Trek Fuel EX 5.

[Bill]

Kind of reminds me of a skewer end?? Or