A car makes a hissing noise each time the A/C system and engine are turned off.

A refrigerant leak doesn’t typically make a noise, except when the refrigerant blow out of the leak. This noise would happen suddenly and would not have a hissing sound. 

Refrigerant leak noise.

On most systems, when the engine is turned off after the A/C system has been on ,a hissing sound will be heard as the pressure within the system equalizes. This is a normal noise. A refrigerant leak doesn’t typically make a noise, except when the refrigerant blows out of the leak. This noise would happen suddenly and would not have a hissing sound.