A stoat can get inside your chicken house through ventilation/mesh openings etc... I lost 5 young hens and a cockerel last autumn to what we think could only have been a stoat - as the house was shut, all birds inside, no mess outside. Very little blood and only small wounds on the birds. Must have been hell in there though, the place was in a total mess... no sign of the stoat though, but I know they are about....
I would have thought it is almost impossible to have your chicken house safe from them at night, unless you have no vents or similar, in which case your birds would die from pneumonia pretty pronto...