The electronic leak detectors can be somewhat misleading because they will "sound off" when they sniff moisture. There may be alot of moisture in the heater/evaporator air box.
My GT had no refrigerant in the system when I bought it. Of course, I checked for leaks and was CONVINCED that I had a leaky evaporator based on the reaction of the electronic leak detector when I "sniffed" the air outlets. Not so. I took a chance and just evacuated then recharged it. The system has held the charge for over 3 years!
My suspicions were correct that the shop that did the clutch job for the P.O. opened the A/C system to drop the cradle.
Bottom line is don't be misled with an electronic detector.
If you do have a leak in the evaporator, you may be able to "see" the UV dye if you shine the light up the condensate drain.