can't you see it's a black round rubbery thing , must be a tyre !
helpful bit , is there a hole in the middle on the other side ? if so , i would think a mount twixt bike and a panel .....or someone has dropped it whilst working on the bike before you !
If on the other side there is a hole in the centre ,There is something like that under the seat pushed onto a pin on the frame and the plastic housing where the tool tray sits sits on it. If you lift the tray out out these can fall off.