I was rereading Killer Market by Margaret Maron. In this book furniture buyers and reporters and others are admitted to a complex of buildings housing offices and showrooms for a set of sales-related parties, as part of a major furniture show. One person P is apparently taking photos from a hallways of new furniture designs through windows into a closed showroom. Another person E comes from inside, and demands the film, threatening to "stomp your camera" if the film is not surrendered. P does surrender the film with an attitude that suggests that he felt he had been caught acting wrongly or even illegally.
P was standing in a hallway to which he had apparently been lawfully admitted, and was not trespassing. (At least let us assume P was admitted lawfully.) Could an employee or owner of the showroom such as E lawfully demand the film, whether under a trade-secret theory or any other. If P had said "Sorry public hall, no reasonable expectation of privacy" could E have taken any lawful action?
The book was set in 1996 or 97 (copyright 97) in High Point, North Carolina, where there is in fact a large furniture industry, and a semi-annual "market week" attended by buyers and sellers from all over the world.
Edit:
Based on events in the book it seems that people admitted to these shows were not asked or required to sign any agreement, including a "no photography" agreement. At least the one person who is a PoV character is not asked, and no one mentions such an agreement at any time. Let us assume that there was no such written agreement. I do see that if there were such an agreement, it would have been enforceable, although not (lawfully) by a threat to do property damage (destruction of camera.