If by some odd chance they didn't get one of their anit-theft tags de-activated at the register and it set off the alarms on the way out I would just continue on my merry way. The receipt in my hand says that what I have in my shopping bag is my property and, unless they have a police officer there with a properly obtained search warrant, they have no business looking through my personal property.
Please consult with an attorney before following that course of action, you'll save yourself a whole lot of trouble.
Attorneys do not need to get involved with receipt checkers unless it is agreed to prior to the transaction. Once the transaction is complete then the item along with the receipt is your own personal property. The only exception would be Costco.
Boy, I bet you are just a bundle of fun at the Public Library.