My daughter on occasion looked after spca foster kittens. One timid one found a hole in the floor boards under the kitchen cabinets and disappeared down like a rat down a drainpipe into the cavity. It would emerge in the middle of the night for food and cat litter box - we'd hear it and investigate but would flee as soon as it spotted us. In the end I constructed a cardboard flap triggered by a long string so we could cut off its escape before it saw us. Took a couple of days to catch the little bugger.

By the time it was old enough to adopt it was starting to socialise with people.