What is the meaning of [Finding’s keeping]

If this were taken as a general principal, it would be morally and legally
wrong, but we must have a sense of proportion. To pick up a purse in the
street and not hand it over to the police is dishonest, but should we come
across some trifle such as a stub of pencil or a safety-pin we shall harm
nobody if we slip it in our pocket saying, ‘Finding’s keeping.’