It is not illegal per se to have cameras or employees watching behind a 2 sided mirror in a dressing room at a store if the depiction of what is being seen is not being transmitted for purposes beyond to protect the store against theft. One condition is that there must be a sign in plain view warning the customer that he or she may be subject to viewing while in the dressing room.That way the customer has the option of proceeding into the dressing room and being aware that he or she may be seen by third parties. The key to your question is that written notice of the possible observation of the customer needs to be given first.