One thing to also consider about the evap, is it is literally being rinsed off when you turn the AC on. I used dye ran it till it all leaked out, when I just looked in the box like you did I didn't see anything, when I pulled it out I seen faint reaidue. Condensation is constantly forming and dripping down and off the evap so it's rinsing off the dye as it runs and the leaks are usually at the bottom, where you'll see most of the water run off. You took the cove off, you are 2 clips in the engine bay coming out of the expansion valve and one cut in the plastic under the glove box away from just pulling it out and seeing for yourself. Pull it and see with your own eyes to completely rule it out.