Wounds on female mice from being cleaned?

Hello!

We are getting worried about a couple of our pet mice. We have 7 female mice that have all been together for almost a year and get along well most of the time. Recently, we've noticed that two have wounds on their upper backs and shoulders. We can't think of anything it could be from aside from another mouse being too rough when cleaning each other. We've never seen them fight. Should we be worried? They don't look infected at all.

Any input is appreciated! Thanks!