Tattoo in the ear or chip under the skin that can be read by any veterinarian.

That's where i'd head, if a cat that would linger several days in an area, with signs of sickness, dehydration or malnourishment. But this one was super fluffy and clean, it didn't cross my mind a single second it would be stray.

But like i say, different countries, different rules. In Australia it would be caught immediately because of local laws.