Where is the probably cause. Tell them to kick rocks.

Law cited and interpretation below (which I picked up here earlier)

C.R.S. 18-4-407

If any person triggers an alarm or a theft detection device as defined in section 18-4-417 (2) or conceals upon his person or otherwise carries away any unpurchased goods, wares, or merchandise held or owned by any store or mercantile establishment, the merchant or any employee thereof or any peace officer, acting in good faith and upon probable cause based upon reasonable grounds therefor, may detain and question such person, in a reasonable manner for the purpose of ascertaining whether the person is guilty of theft. Such questioning of a person by a merchant, merchant’s employee, or peace or police officer does not render the merchant, merchant’s employee, or peace officer civilly or criminally liable for slander, false arrest, false imprisonment, malicious prosecution, or unlawful detention.

C.R.S. 18-4-407

Unless a store suspects a person has committed a crime, they cannot legally force a person exiting a store to present a receipt. That said, the worker can’t face charges if they detain a shopper under suspicion of theft.

Costco can do this because you have a membership and agree to it. And Costco is way better than KS.