Originally Posted by
dsbrantjr
That looks like a plug for a rack mounting hole. Look on the other side of the bike to see if there is one like it there someplace.
Edit: Did it perhaps come out of the end of one of the rack tubes?
+1 on the end of the rack tubes. The open end can lead to water inside it and consequently rust, unless it’s aluminum. If there’s no other one on the other side, I’ll assume it’s missing too. Measure the openings and look for rubber plugs somewhat wider than it for a snug fit replacement.