(UK based). My question refers to the situation where you are using the "scan as you go" system in which you scan items and pack them into your bag as you go around the shop, and pay at the till before you leave. When using this system you agree in the terms to random rescans, in which your items are occasionally completely rescanned by store staff to see if there is anything you didn't scan.
However, this seems to imply that the shop staff have the ability to go through your personal bag, which may have your own belongings as well as your shopping. As far as I am aware here the store has no recourse to use citizens arrest powers, as there has been nothing to suggest you have broken the law, so they have no power to search your bag or detain you for a police member to search your bag. But if they have no power to search you, surely the rescan system will never catch potential shoplifters, as any unscanned items can be kept concealed in your bag.