The Police used to ask me to attend after a vulnerable person had been burgled, or had an attempted burglary. One of the things I often did was set up a simple alarm in their shed that would activate when the shed door was opened. If you new the alarm was set you could partially open the door and unhook the cord that set the alarm off. If you didn't know the alarm was there and the pin was pulled from the alarm, there was no way that you could enter the shed and reset the alarm ( the noise was ear piercing).
The alarm is activated by a pin being pulled from the device.
Being in the trade I would usually buy around 20 at a time and would give them away to ladies to attach to there handbag and a wrist cord. When your handbag gets snatched the last thing a thief wants is an alarm going off and the victim screaming ... help .. thief !
This type of alarm would be ideal for your problem and could easily be set with a couple of hooks or adhesive.
Can I ask what you are hiding in the plant pot .. it's not your key is it ?
http://tinyurl.com/qjqynjc