about a year ago had a cat appear at our house, it took about 6 weeks for me to be able to get close enough to be able to determine if it had a collar, it had taken a few weeks to get a good look at it! It had been living under the decking and in the bushes around the house.
In the end got a cage off cats protection, trapped the cat and felt awful doing it, but managed to get it reunited with its owners.
It transpired that it had wandered 3 miles across the fields, and in the end was away from its owners for 8 weeks. But it did get back to them, it was fit and healthy, and despite living outside in January in the snow, it was fine.