Thats ok. Don't feel bad, most people don't even know what their own car alarm sounds like. So don't worry about it. It was probably a false alarm anyway, just enjoy your meal and worry about it when you leave.
Not only do YDI, but this should prove to you that your car alarm is 100% worthless. If someone had broken into your car to steal something, they'd have it and be long gone. If they were stealing your car, they'd have disabled your alarm 14.5 minutes before you noticed anything.
Car alarms only do one thing. Piss off your neighbors to the point that they want to to put a brick through your windshield because your alarm goes off every time a cat brushes against your car.