Customers.

There are two men who regularly come into the bookshop. I feel sure that they are up to some mischief. They have that alcoholic sway thing happening, and they hang out by the art books. One wears an enormous jacket, no matter if it's hot, the sour stench of him spreads throughout the entire shop floor. They choose maybe 10 books, and then go and sit at the table and chat and look at the books. The jacketed one takes his coat off and spreads it wide so that the table is obscured. They get up and stagger off. They return again and again throughout the day. I try and count the books on the table before and after they leave. I can't tell if anything is missing. Our art books are security tagged, they never set off the alarms. Our security guard has watched them closely and can't see that they are doing anything.
Could it be that they are two down on their luck street guys who need a regular fix of art?