That the insides of shoes is interesting, that's normal, but today he was more interested in the outside and he was very keen on investigating the interior of my backpback. He also wanted to go out immedieately and after a short while back in again and search the area closest to the door. And repeat. He also seemed extra eager to mark territory outside.
There's a reason, for on my way home I met another ferret.
I'm not sure if this was a territorial defensive behaviour or if he'd want to meet another ferret again.