If you had the A/C on high, the compressor is pretty noisy from outside. As soon as you opened the hood, it triggered the engine to start. Opening the hood forces the engine on for maintenance reasons.
Next time you think it's running, go to the rear and put your hand in front of the exhaust pipe and you'll know if it's really running.
Next time you think it's running, go to the rear and put your hand in front of the exhaust pipe and you'll know if it's really running.