Sometimes, no matter how hard we try to limit exposure, it can still happen. Those little buggers can certainly be a mystery. I had them one time a few years ago and it was exactly like you describe. Only in one enclosure, on one snake I'd had long term, and no reasonable explanation for it. Needless to say I tore my house apart cleaning and treating everything in site for the next 2 months lol!

All it takes is one female to hitch a ride.