If I were you, every time you hear a small 4 stroke engine start up I would hop on another bike and haul a$$ towards the sound. And you could also tell all of your friends and family to look out for it.
And if you find the theif, you could have one of those mini-bats that they calm down large gamfish with hanging from your wrist. http://tbn0.google.com/images?q=tbn...www.kayakfishingsupplies.com/catalog/8283.jpg
I have an idea !
Can one of those gizomo's be made to plant the bikes with a beeper thing, I think it's called a sattelite tracker. Cars have them, I think it's mandatory on boats for sea rescue.
That way if bike is stolen, it gets tracked down by sattelite.