compact vehicle revolution

There was some kind of vehicle revolution, or a really funny prank, going on in front of our house last night. Around 10 I went out to my car to fetch my Nalgene. My car, Lauryn's car, and a car I didn't recognize were parked in front of our house. Unlock, beep beep, open door, grab bottle, shut door, lock, beep beep. As I walked back to the house, a car went Beep Beep BEEEEEEP! It sounded like my horn but I had no idea why my car would be beeping at me after I just locked it and reset the alarm. It stopped, so I kept walking, only to have a long series of beeps begin. The minute I turned back to the cars, it stopped again. I unlocked and relocked my car for good measure and continued onto the porch. Where, you guessed it, some sort of beeping revolution happened again. But anytime I got near the cars, it stopped. Eventually I gave up trying to solve the mystery and headed inside to the sweet melodious sound of car alarms.