More specifically, It's the pressure equalizing in the A/C system, Pressure is relieving through the orifice used for the pressure drop.
An experiment you can try.....Start car with A/C off.....turn car off....No Hiss right?
Start car and turn on A/C....let it blow till it's cool then shut off the engine.....Hear the hiss?
I bet it's back.
It's a totally inherent quirk for almost every vehicle, Totally normal...