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The Self-Checkout Nightmare May Finally Be Ending
(gizmodo.com)
This is a most excellent place for technology news and articles.
Legally they can't force you to show your receipt. But refusing to show it could constitute probable cause for employees to detain you while they sort things out(shopkeeper's privilege). Detaining you could constitute false imprisonment, though Colorado courts recently ruled Walmart not liable in that regard. The store could also choose to ban you.