I've seen battery-powered intruder alarms which either clip over the top of the door to hang on the inside or hang on the inside door handle. If the door moves the alarm sounds.
I don't know whether there's one available with a remote control to set & de-activate from outside. On the one hand, it would be nice to have a deterrent for anyone wanting to lie in wait, but on the other hand it would scare the living daylights out of the maid!
ap.