When they charge car AC systems, they can put in stop-leak and o-ring conditioner additives along with the refrigerant and oil that basically make the seals more pliable and/or seal any small leaks that may have happened. Sometimes they have marker dye included as well, so that any leaks that don’t get sealed can be identified, usually with a UV light.
I’d bet that the last time you got it charged, they used one of these products, and it sealed your small leak.
I keep my cars about ten years, and I’ve never had an AC issue. I can’t think of any car in my family or extended family that I’ve known to have an AC issue, so I question whether the occurrence is as common as the OP observes. Or maybe I’m just lucky.
Auto manufacturers and engineers strongly recommend against using stop-leaks in a/c systems (and radiators, and pretty much anything else). They sometimes work to stop leaks, but they also plug up the system generally and will likely do more harm than good.
If your car is an old clunker and you’re not willing to spend more than a few bucks on it, then it can be worthwhile to take a chance on these since the worst they can do is stop your a/c from working and it doesn’t work anyway, but if you plan on keeping the car a while and would be willing to spend what it takes to make the system work if necessary, it’s strongly discouraged.
If your mechanic has put a high-risk thing like stop-leak into your system without telling you, then you should switch mechanics.
All I was saying is that if he typically had to get it charged annually, and then suddenly it stopped, and he hasn’t had to do it since, something like one of these products is probably the answer.