I don't want to return a possibly lost cat
Why don't you want to return the cat?
I want to keep it as a pet.
You should never take an animal without knowing if it has an owner. You should post online about it and see if an owner responds.
What do you mean? I'm going to keep it.
There may be an owner out there who is looking for this cat. If you do your due diligence and search you may be able to help someone reunite with a pet that they love.