The children across the road from me were looking after my neighbours' very elderly cat while they were on holiday. One day, the kids arrived at my door asking if I had seen Snookie. I hadn't but I knew where - under a bush in my garden - she liked to lie during the day. Sure enough she had gone there to die peacefully. 
A happier story - my son's partner got two new kittens last year. Early this year, one of them, almost fully grown, just disappeared in the space of a few minutes. She put up posters round the neighbourhood and delivered flyers round the doors. She had given up hope when, five weeks later, she had a phone call - 'I think we have your cat in our garden.' And there she was, none the worse, and so glad to be home that she's hardly moved out of the garden since then. No-one knows where she was all that time and she's not letting on.