The air conditioning has a slow leak.
There is specialized equipment for finding leaks in AC systems. One way is to add a fluorescent dye the the system, run the system, and check for a leak with a black light. There are also electronic tools to detect the refrigerant from parts that are impossible to see. Only professionals should service the AC on your car. Improper practices could damage the compressor and become very expensive to repair. In order to have this issue properly taken care of, have a certified technician inspect and repair your leak for you.
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