How to avoid accidental cat-napping?

Check if the animal is tattooed (usually on the inside of the ears)—although tattoos are being phased out in favor of microchips (the latter being the only recognized form of identification in the EU if applied from 2011 onwards). If it is, a local vet should be able to see if the animal is registered.

Take it to a vet to see if it is microchipped and registered. Microchipping is required by law in some countries (Lithuania being the one I know of) and an unchipped animal is possibly less likey to be reclaimed by the owner. In other places, while not required by law, it is still very common.

Repeat the collar exercise with a cheap but strangulation-safe collar from the pet shop, and attach a capsule with your contact data to it, as well as an explanation that you believe the animal to be ownerless and asking the owner to notify you. The cat might have ripped off the paper collar, or it may mean that the owner simply doesn’t care and ripped of the collar without calling back.

Other than looking for lost-cat posters, you can also put up found-cat posters with photos, description and a contact number.

Check your local laws, most likely the Civil Code or equivalent of your legislation regarding found animals. A local vet might know the law and procedures. For example, Lithuanian law would require you to report the found animal to the police or city municipality within three days, after which the owner has two weeks to claim the animal—if he does not, you are its legal owner. Things may be different where you live, but similar principles would apply.

It depends on your location. Check the local animal rescue shelters and regulations.

The first thing to do is to take the cat to a vet to see if he is microchipped. Most house cats in UK are microchipped, so a vet can contact the owner on your behalf. They would normally do it free of charge, but it depends on the available vet as well.

The second thing would be a sturdier paper collar, or easy break collar with a name tag. Still, make sure that the cat can break it away if he needs to, otherwise he might strangle himself if the collar gets stuck to some branch or fence. Easy break collars usually cost around £5 and you can engrave your phone number on them.

The third thing would be to contact your local animal shelter, so that they can make an ad about the cat. People who lose their cats usually go to the animal shelters as the first thing.

I am currently going through a similar procedure with my cat. He was apparently stray, but everyone taking care of him thought he had an owner. They also love to pester different people for food, so it is good to contact everyone who feeds him. A WhatsApp group would be very useful.

I suggest cat-napping it long enough to take it to a local vet and seeing if it's microchipped. If so, you'll know right away whom to call.

If not, take some photos (face, back, and both sides of they differ) and ask any other local vets or shelters if they recognize the cat, and post them on local groups on Facebook, Nextdoor or other social media sites.

You can also try using a more sturdy collar that the cat isn't likely to destroy, either on purpose or on accident.

Failing all that, if it seems to want to move in permanently, just keep an eye out for any lost-cat posters. Spending hours, both night and day, trying to be with you doesn't sound like it is just passing the time while its family is away at work. More likely, it has been stray for quite a while and relying on its friendliness to stay safe and well fed, which is how cats domesticated themselves in the first place. But even stray cats can appreciate a forever home when they find it.