There is a hole in ceiling of the storage units high above the sink and shower of our bathroom (kinda like the song: "there's a germ on the flea on the hair on the wart on the frog on the bump on the long in the hole in the bottom of the sea").
He has found his way into said units and rustles away in there.
One day my housemate opened one of the doors to check it out and I watched in the reflection of the mirror. There he was.
"Freddy you little f*****!"
A little grey head, eye contact and then a scurry back into the roof.