This is an idea intended to solve 3 problems:
stray cats hiding underneath, which is bad in a few ways (provides an unseen spot for them to bolt unexpectedly from into traffic, and a place where they feel free to fall asleep and get run over by the car or hide in the engine compartment for shelter
get killed in there), destructive little marmot bastards who crawl up into the undercarriage and chew on everything, and carjackers.
This device consists of 2 main components. The first is a pair of infrared sensors, one at each end of the car, which are protected by automatic lens covers like those found on many digital cameras. When the vehicle is not in motion (either parked or just stopped) the sensors pop out like tiny ventral periscopes, just far enough to be able to scan the entire area under the vehicle, and barely visible. If anything goes under the vehicle, the sensors will know, and will activate the idea's 2nd component.
The 2nd component is a pair of similarly-protected speakers, also mounted to the underbody, but hidden. Upon sensing an unwelcome presence, the speakers play a recording of a deep, throaty rottweiler growl. If the presence does not immediately beat a hasty retreat (say, within a few seconds) the growling intensifies and turns into vicious barking and snarling.
This is almost guaranteed to keep stray animals out, and probably thieves, as well. If connected to external motion-sensors, this could make the growling start as soon as somebody gets near the car when the regular alarm is activated. Add a slight rocking to the car, and no passerby would even think of trying to steal the car with The Rottweiler hiding underneath.