I'm not sure whether to worry about this...

Hi everyone, when I was petting one of my guineas yesterday I felt somthing like a tiny scabby bit behind her ear. On closer investigation it is like a tiny crusty bit, about maybe 3mm wide by 1mm. You can't even see it because it's not irritated. She doesn't seem to mind me touching it, it isn't inflamed or anything unusual. My first thought was that it might be dried-on ivermectin, since it is at the base of the bald patch behind her ear, just before the fur begins. It won't rub off though, so I have no idea. Otherwise she seems absolutely fine, no missing fur, no other crusty bits, no lumps or bumps. Anyone had this or have any ideas?