Our cats frequently bring us little ringneck snakes - black snakes with a white collar under their chin. Maybe two a month in the summer.

This alarms my wife to no end because we only have indoor cats!

It's probably just a harmless rat snake. Better to have than not for mouse and rat protection (If a snake is getting in so are mice). Sure they alarm you, but otherwise snakes don't do much harm. Mice, on the other hand, can be very destructive.