Q: AC is leaking R-134A but no dye can be found anywhere

So my AC has been pretty bad the last year. Earlier this summer, it was blowing pure hot air. I checked the pressure with a manifold gauge, and it was at 0. Dead empty. I filled it with the specified 600 grams of R-134a, and it worked like a charm... for about a week. Then it slowly became warmer and warmer, until now, there's just hot hair.

I used a dye and checked for leaks, and found absolutely nothing. I even took it to my mechanic to have him inspect it with a UV light, and he couldn't find a trace anywhere. Compressor is engaging when enough R-134a is in it, lines are good. The only thing He could think of is maybe a bad evaporator core. I had this replaced about 6 years at the dealership, so I would find it odd if it was already bad. I also don't smell anything inside the car. There isn't any smell or odor whatsoever when the AC is on inside the cabin, even when it is charged.

Can anyone think of anything? Anywhere I may want to look?

My car has 140000 miles.
My car has an automatic transmission.