Hole on head?

maybe it`s a scar from rodent bite... never leave live sub-adult or adult rodent especially, rat without monitoring..

Thanks dek593 for the comment; however, I never feed live adult or sub-adult rats. I only feed rat pups or adult mice. Now that I think about it; I don't think this came about after a meal; the new ones just showed up one day which is why I was thinking maybe it was bacterial or something that was spreading through the racks.