Help with my little escape artist, please

I have a male ferret that will be 4 in December. All of the sudden he has been escaping to the outside. He always comes and scratches on the front door to be let back in, but it terrifies me that he is getting out, and I can't figure out how he is getting out. Today he was outside for 2 1/2 hours (while I thought he was asleep in my kitchen towel drawer.)Does anyone have any insight on why this has suddenly become an issue? How to ferret proof better? We haven't moved, he has lived in this house since he was 6 weeks old. He gets a bit of outside time (2x a week for about 15-30 min) he is fixed and fully vetted. Just for info I have a female the same age and she doesn't go outside with him. I just don't understand.

I would be looking in cabinets to see if either mice or your fuzzy has made a hole somewhere. They can get through some pretty tight places. I take it he's free roam. Maybe confine him to different rooms to see what one he may be escaping from. 

 
Fuzzbutts Rule!

If you live in a manufactured home, especially, there are probably big holes under the cabinets for pipes.  I wasn't in this place 24 hours and left a bathroom door open and 5 got under the house.  Also check your dryer vent connection and check all window screens for holes.  We had a piece of paneling 18-20 inches high in front of the main door at our old place to keep them from "ghosting" out the door behind us.  That was a mobile home as well, and Alice and Poppy used to get out under the kitchen sink.

Here, I filled the holes with spray insulation foam, but they still will dig at it if they get the chance, so I had to put gorilla tape around the bathroom vanity as well.

THE DOOKS OF HAZARD

I would be following him around when you let him out. This way, you can see where he is getting out. Good luck!