Does the office have its own door? It can very easily be fixed so that at a glance you can be virtually certain the no one is in the room.
We have a room that has cat food out and a litter pan, neither of which we want the dog to have access to. We keep the dog from going through the ajar door by looping a string around the inside door knob, through the opening and around an cup-hook outside the door. The cup-hook is in-obtrusive because it is concealed in the angle of the door-frame and wall.
The length of the loop of string is such that no one in the "cat's room" can fasten it outside. If the string is in place, no one is in the room. One might argue that a partner in crime might fasten the string (I actually use artificial sinew), but I am not that paranoid.