strange beeping coming thru speakers

[08altima35]

went to the mitsu dealership today that was closed to check out a used shelby500 mustang and my passenger and i got in the car to leave and just as i started the car there was the most random double beep coming from the speakers. i turned the radio up and it happened again just louder. no dash warning lights or anything out of the ordinary, just a beeping sound. i killed the motor and re-started it and everything was fine... so i think.anyone had this happen to them and have the slightest idea what it was?thanks in advancemaybe my a/c was just telling me get me the hell outta here, but prolly not.

[generic808]

Did you open the door when the engine was on? It makes two beeps if you do that. Or do you have any maintenance schedule on your comp? Or it could be low gas? Those are just a few suggestions because they all sound a double beep.