That was the official diagnosis. I said, "So it is just gone by magic?" and was assured that this is not uncommon at all! Maybe with Mitsubishis; I have never had to mess with the A/C on a car before 10 years or so. I think I remember reading some posts on here about others' cars that came underfilled from the factory so it would be worth checking out.
Eh, I should not be expected to top up the A/C refrigerant periodically, just like the vehicle should not consume coolant or oil which would point to a mechanical problem like a head gasket or bad rings. It isn't really possible to get the refrigerant level correct with those combo can/gauge things from Walmart, plus it is best if they pull a vacuum on it and get the air out instead of just cramming more refrigerant in there.