Is it possible to detect and repair a refrigerant leak in my HVAC system?
Working on the refrigerant lines requires special tools and skills that most DIYers don't have. And you'll likely need a license to get additional refrigerant. Therefore, fixing the problem will require a professional.

I'm a bit surprised that you didn't say he made any attempt to locate the leak for you. I would start searching for the leak by applying some diluted dish soap over all the joints in the coolant lines. One other place to check are the caps on the valves. If you don't see any bubbling, then turn on the AC and see if any of the joints start to bubble. The vibration and pressure changes may set off a leak. Some of these joints may be within the inside and outside units, so if you don't feel comfortable disassembling these, you can get a professional to find the leak for you. Otherwise, be very careful working around the coils, the metal fins are very thin and easily damaged.

Since it was only low, I would wait for a year and have the system serviced before the next cooling season starts. They can check the levels and let you know if the levels dropped. If they find it dropped again, they should be willing to find and fix the leak for you. Since the first person didn't seem to want to do this, you may want to get a different company to service the unit next time.