That looks about the right size for a broken off belt keeper and with the scuffing through the paint definately looks like something rubs against it,
If it is a belt keeper then the belt probably jumped off the pulley that the keeper was supposed to be preventing the belt parting company with.
Your owners manual should have the parts break down so look at all the pulleys and the keepers ( some times called a guide ) then check what you got left on yours.
Expect to replace the belt as well.
