Yeah the trick is to put your reusable bags in the trolley, with a second reusable bag inside them. Put your groceries in the inner bag, and the items you’re going to steal in the outer bag between the layers of bag as you shop.

Then when you get to the self checkout, you unload the inner bags onto the checkout, scan as normal, move your trolley to the other side with all the unscanned items hidden between bag layers, pay, load the inner bags with the paid groceries. They’ll never even see the stuff between bags after you’ve unloaded for scanning, and if they look into the bags it still looks empty.

… Or so I’ve been told