We had the same problem in JHB, resulting in the same solution. We found, however, that at night the sensors were fine and didn't tend to give false alarms.

Now we have a new swanky system called Roboguard, that uses two beams per sensor and seems to be a lot more stable against false alarms - typically only monkeys and fleeing GOC*s set it off. Failing that, shark tanks!

(*Giant Orange Cats)