I replaced at work our 3 ton rheem package heat pump about 4 months ago due to some electrical problems. I had it sitting in the yard so I could turn it into a trainer model for the guys. Yesterday, tried to recover refrigerant and it was bone dry. I couldn't pull an oz out of it. There's no signs of any leaks anywhere and it's in a place where no one could tamper with it. I put a leak detector on it also. Today I charged it with nitrogen at 150 lbs and left it overnight but it looks like the pressure will hold. Any ideas what happened to the refrigerent? I know the oil will hold some refrigerant but 6 lbs of refrigerant?