Only true for a small shop where the shopkeeper can see all entrances and is able to pay attention to that enough of the time. Otherwise you need video cameras

Only true for a small shop where the shopkeeper can see all entrances and is able to pay attention to that enough of the time. Otherwise you need video cameras and AI... or perhaps spray every incoming customer with a source-identifying powder or smell - hmm, maybe that explains Abercrombie & Fitch!