One of mine got under the floorboards via a little hole under the gas fire - I saw him go through but couldn't get to him in time.
I heard noises under the bathroom that night so knew he was under the house.
I was able to pull back the carpet in the living room and lift a floorboard there, propping it into the space so he would have something to climb up (there was between a foot and 18 inch gap under the floorboards). I then laid hamster choc drops at intervals up the plank and put a bowl of food at the top next to his open cage.
Next morning, all the choc drops were gone and the food bowl was empty but no hamster in the cage.
The next night I repeated the same process, but this time slept in a sleeping bag next to the ramp. Nothing, but you could occasionally hear him rustling and scratching under the floor in other rooms.
On the third night, the noise of him rootling in his food bowl woke me and I went from being asleep to my hand grabbing him before I consciously knew I had heard him!
He was really dusty, with filthy nails. Back in his cage, with one cheek pouch still full of food from the bowl, he immediately stuffed the other with bedding and waited by the cage door. Clearly now packed ready to return to freedom under the floorboards!
It always made me laugh how careful I had been with him, shielding his cage from draughts, avoiding or limiting certain vegetables, washing his water bottle daily, etc, when he was plainly perfectly happy making his own way in life, marauding around in the cold and dark under the house, gnawing on insulation tape.