If people at your location are having problems following normal procedures with regards to using the water closet, an interesting and humouros solution by the DIY Hacks website. They've created an alarm that checks that three actions have been completed, and if not a bright matrix of LEDs flashes with an appropriate warning.
Two piezo sensors are used to detect hte actions - the first detects vibration caused by water flowing through a pipe to the cistern, and another detects water flow to the hand basin. Then a reed switch and magnet indicates to the Arduino that the door has been opened. Thus if sensor two (the hand basin) hasn't detected water flow - the alarm appears. A demonstration is shown in the following video:
A great project to use as a joke or for more serious purposes - and all the detaila are in the Instructable page. And for more, we're on facebook, twitter and Google+, so follow us for news and product updates as well.
If you're interested in detecting vibrations or working with making sounds and tunes for various reasons, we have a neat little SOUND: sound and buzzer module:
It can be used as a noise-maker driven by your microcontroller for audible feedback of events, and it can also be used as a knock-detector input to sense events and react to them. Includes a built-in 1M resistor to allow the piezo element to detect shocks. For more information and to order, please visit the product page here.