IMO, if you let your cat outside unsupervised, you can't be upset or surprised if they end up living with someone else. Frankly that's the BEST case scenario. (Source: grew up with indoor/outdoor cats and a lot of our cats moved in of their own accord from other people's houses, and a lot of the ones that started at our house also had other families--wasn't abnormal to not see a cat for days/weeks on end)