My ferrets sleep wherever. They are certainly funny creatures.
The *usual* reason for them sleeping on the floor is *usually* if they are warm but not always.

How old is the fert?
How long have you had him?
Has he ALWAYS slept in a hammy?

If you have had the fert for quite a while & NEVER seen him sleep outside of a hammy I would keep a close eye on him, make sure he is eating & drinking good, poops look normal etc..

If you have any ?'s at all feel free to ask me, here, pm or email.

I currently have 5, maniac is in my sig.