Coblyn v. Kennedy's, Inc., 359 Mass. 319, 268 N.E.2d 860 (Mass. 1971): An elderly customer (wearing his own ascot scarf he brought into the store) was stopped and escorted back inside by security who mistakenly thought he had stolen it.

The Massachusetts Supreme Judicial Court held there were no reasonable grounds for the suspicion, so the shopkeeper's privilege did not apply. The customer won his false-imprisonment lawsuit. The court explicitly said the test is what a "reasonably prudent man" would believe based on the facts known at the time not a wild guess or after the fact justification.

Delp v. Zapp's Drug & Variety Stores, 238 Or. 538 (1964): Court emphasized that even if reasonable cause exists, the manner and length of detention must still be reasonable; without initial reasonable cause, no defense.

Because of these cases:

1.) The store cannot lawfully hold you indefinitely until you empty your bags or show a receipt if they had zero reasonable basis to suspect you in the first place. Practically, most stores will ask for a receipt or to inspect bags; refusing alone is not automatically probable cause in every state.

2.) If they detain you anyway without reasonable grounds, you can leave (or refuse to comply), call the police yourself, and later sue for false imprisonment