Do go and check for that distant meow again, try to track it, where do you hear it more loudly??
Do you have a cellar she can have sneaked into, do the neighbours have an open basement window she can get into but not out of? A car engine block, maybe? Cats do tend to cuddle up on the engine block of a car and get stuck....
Do you have a boiler cupboard, cupboards in the bathroom, or something like that?
We have a combination boiler with an expansion tank, and one of my cats had once managed to wriggle behind that tank. Only, she didn't know how to get out without burning her paws, as the hot water had been used since she sneaked in, and the pipes she climbed over to get there were now all hot.....
There are so many places a cat can hide, and just when you think you know them all, they discover a new one..