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.
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.