The fridge alarm had a melt down.
I came home one day to find that it was constantly on. It wasn't even beeping periodically, it was just a constant drone. This was a little odd because the batteries were dying. I presume it was beeping for most of the day and so the batteries got to the point where they were on the edge of being able to power the relay and so the relay current stopped the 555 from getting enough power for long enough to do the timing. Just a guess.
So anyway, it had been dodgy for a while, I had to regularly adjust it to keep the switch in line with the big ugly box that pushed it. I think it must have got stuck before I left, the Peterhouse cleaners presumably found it beeping and tried to stop it as the big ugly box was at an odd angle and so was the fridge. At this point I removed it from the fridge to make improvements.
This problem is usually solved with magnets and reed switches. Normally closed reed switches are not common but are still fairly easy to source. I got an SPDT reed switch and a "heavy duty" reed switch kit. The kit had a SPST normally open switch in it. This would obviously be useless for this application as that would cause the alarm to only sound when the fridge is shut. I replaced the switch inside the box with the separate one that I bought and had an ideal solution that required no physical contact between the two parts of the detection mechanism. Hooray!
I also added a kill switch for when things go wrong.
To summarise; the detector is now more reliable as it is a no-contact solution. There is a kill switch so the cleaners aren't driven mad if it fails (it'll ever be needed). Current is still only drawn when the fridge is open so that the batteries will last a long time.
Some pictures... (click for bigger)
Switch kit with old switch installed...
Close up of old switch, out of focus :-(
The new switch, with added magic... (bent leg was removed, this was the normally open contact)
The emergency kill switch was stuck to the top and its purpose made clear...
This image shows the reed switch kit mounted to the side of the fridge (with new reed in place inside)...