Athley - what you need to do is buy your cat a little jumper from somewhere like Pucci petwear, then your cat will go out in the winter. Her friends will beat her up, and all the big handsome Tom cats will snigger at her - but it will be nice and cosy.
As Mr Nose says - cats are capable to squeezing through gaps and jumping great heights. And it may be that all the cat has to do to get into his/her house is to sit on the window ledge and scream.
But you have to put your mind at rest because, like the rest of us, you love and care about animals. Investigate further by checking at the relevant house or at one of the neighbouring houses.
But remember our 'dumb' friends are much more intelligent than we give them credit for. I fell for my late cat's starving cat story many a time (the 3am wake-up call) -he would run down the stairs with his big belly bobbing off each stair in his race to get to his bowl which was full, but he wanted new stuff.