It's almost surely leaking if they have to keep adding it..
Well, unless there is a restriction somewhere and they "think"
it's leaking out.. But it probably is leaking.. They need to use a
refrigerant leak detector. And yep, it's probably in the evaporator
coil most likely. But sometimes the compressor seals can start
leaking, so that's a possibility too. Dyes and black lights are for
pikers.. :/ Pro's use leak detectors to sniff the leak.
I don't do car work much, but I do HVAC work. I have three different
leak detectors. I still think the old GE plug in the wall model is one
of the best ever made.. I have two of those. But I also have one of
the new fangled models which runs off batteries. It's basically a piece
of junk compared to my trusty GE style units.. It works OK, but it's not
near as precise when you start narrowing down a small leak.
If they can't see any dye anywhere up front, it's probably in the evap
coil.. Which is a PIA to get to in most cases..