You can find these for dirt cheap at Dollar Stores or online and home improvement stores for not much more
They are a very basic and simple battery opperated door / window alarm. The sounder has a simple on / off switch on the side. If the small magnet section is moved away when in the on position, it produces a surprisingly loud 110 db sounder. You just need to turn off the sounder before you move the magnet away.
There are all kinds of creative ways you can use them to protect bicycles and other items.
Lock up the bikes and place one of those cheap blue tarps around them, securing the tarps via Bungee cords. Place one of these alarms "with the magnet next to it" under one of the folds so they won't separate unless you start trying to pull the tarp off. As soon as someone even starts to pull the tarps up they will sound the alarm. Unless they think there is no one within 1000 yards to hear the alarm, they won't stick around long enough to try and find the source of the alarm. Their only concern will be to get away before anyone sees them there.
I like the idea of placing the bikes in the tent when your away. Use one of the Self Contained Motion Detector / Siren and Remote's you can pick up pretty cheap.
After you zip up the tent, you use the keyfob remote to arm it. If it sees motion it will sound the alarm. This will protect bikes and all contents of the tent You just deactivate it with the keyfob before you open and step into your tent. Battery operated and should last for several weeks before worries of needed to replace the batteries.
