Putting it next to the original horn makes it really easy to clip the alarm wires from under the car before attempting a theft. This is what I do to assholes who let their alarm go off every night.

I would put it on the firewall behind the engine, somewhere you have to actually go under the car with a flashlight to reach, and can't get to easily with the hood open. Like ^^^ said, under the battery tray is damn near impossible to access.