If there were car alarms, would it be applied like a car part? This could point out that only working cars had them still active. But then again, you would need activate the controls by getting in the front seat to turn it off. Or by wasting 30 minutes taking it out to shut it up forever. This might force you avoid getting the zombie AI to crowd over usable cars.
If it wasn’t a car part, how can you differentiate a shopping cart, steel frame, or wreckage from having alarms? This is a technical problem which I have no experience in so I’ll leave it at that.
Car alarms could be a chore to turn off and avoid. They might be useful noise-emitters, don’t zombies follow you and your scent instead? I personally don’t use noise emitters and get by fine just killing zeds with melee or using a pheromone anyways.