What is a sign of a potential refrigerant leak in an A/C system?

A sign of a potential refrigerant leak in an A/C system is a hissing noise when the car is off. This hissing sound often indicates that refrigerant is escaping through a small leak in the system. When the air conditioning is operational, the refrigerant is under pressure, and if there is a breach in the system, the escaping refrigerant can create a distinct hissing or bubbling noise as it exits the pressurized area. Noticing this sound can alert technicians or car owners to the possibility of a refrigerant leak, prompting further investigation into the A/C system for repairs.

The other options do not specifically indicate a refrigerant leak. Expected temperature from the vents indicates that the A/C is functioning properly, constant fan operation refers to the behavior of the blower motor, and decreased engine performance may relate to various other issues but does not directly signal the presence of a refrigerant leak.