Tip-taker detector

The tip-taker detector is a standard penny dish which stands on scales sensitive enough to detect the weight of a note/coin accurately. The scales would be rigged up to some sort of processing unit, which would set the new value for the weight of the contents every time a new contribution is made.
The idea is to detect whenever somebody is leaving your tips (US: concessions, right?), or stealing them.

On a change in weight, the program would re-evaluate the weight. If it increased, either nothing would happen or the system would play a polite "Thank you" sound. If the weight decreased however, the program would initiate a thief-catching system.
This could be anything from locking all the doors around the dish to administering an electric shock to the thief.

Of course, some sort of failsafe would be included. A button, for example, that shopkeepers could hold down while removing tips could temporarily disable the system.