We found our cat young and starving, being left alone outside.
But he was used to be outside and just locking him in would be cruel.
He mostly happily hides in a hole or some cellar around, when he is outside anyway.

We tried giving him a safety collar, but he was already like 6 months old, when we found him and wasn't used to one. So he always got rid of the safety ones and we were too afraid that he should strangulate himself with a non-safety one.
He is chipped. So I hope this is enough, when someone catches him.
But he is very shy with other people and it worked out for 13 years now I'm different areas of the city - sometimes, in his young and wild years, he was gone for 2-3 days, but he always came back happy and hungry :-)