at home i have a dog who growls at anything so he protects my bike as it is locked in the garage.
However i often leave it round at my mates house where he has a shed inside a shed and on the door of the inside shed we always line up LOTS of party poppers and if the door is opened BANG! if this does not wake him up the we made something else this week.
At school we had to build something with a mercury tilt switch so we rummaged around in the cupboards and found them. We wired these onto a circuit connected to a siren and this is LOUD (can be heard from more than a mile away, probably illegal) so we rest these on our sump guards and if they move.......