LONDON—A man strolled down the candy aisle of a grocery store in England last month, picked up a bar of chocolate and stashed it in his back pocket. He wasn’t stealing. Specially equipped surveillance cameras were tracking both his body and the products he was taking off the shelves, to help him pay for them.

Tesco PLC, one of the world’s largest supermarket operators, demonstrated this technology recently to investors, labeling it as one of the retailer’s big ideas for making shopping at its physical stores more convenient....