Can the air-conditioning system be low on refrigerant with no leaks?

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I recently had my car air-conditioning serviced, and was told no leaks were found in the system , but it was low on refrigerant. They evacuated the old refrigerant from the system and put in new. I had thought that you could not lose refrigerant unless there was a leak in the system. It had been approximately 2 years since I had it checked.