even though its sealed, its still got O rings and other things that fade with time, depending on use and climate.

also the condenser is in front of the rad, so it can get a pin hole leak from road debris. doubt this is your problem though, cause you wouldn't have been able to recharge it.

theoretically, if you had enough palm tree seeds in there to clog the evaporator, so it would work in-efficiently enough for how small the system is, your compressor could have "boiled" off some of the refrigerant. but the compressor would have needed to get pretty hot, and there would have to have been enough clogging to cause little to no temp change through the evaporator to keep the pressure up (plus get past the pressure valve) to do that.. the system would have been kicking off and on for a while.. (similarly to when the refrigerant is low, but consistently since the pressure switch would keep it running)

most systems need a slight recharge after 5 years, again with use/climate, anyhow. just from the system burning the pag oil out of it. i would say if you refilled it with a bottle that also had O ring conditioner, you would probably be ok for longer than you think. but the presence of a leak is possible..