Hi.
I've come across a few leaking evaporators before, usually they have a small crack somewhere in the core which makes it difficult to find because its in the heater box and the leak doesn't show straight away after you re-gas with dye but sometimes you get an AC gas smell in the cabin after a few days.
Another tell tale sign of a leak is a patch of oily
(oil is from AC system) dirt/dust in one section of the system (eg. patch of oily dirt in one section of a AC condenser or pipe flange connection with an o-ring) but you can't use this method since you can't see the evaporator because its hidden in the heater box.
If it was re-gassed 3 weeks ago with dye, your AC specialist should be able to find it this time using the UV light and electronic AC leak detector (probe/device that beeps when ac gas is detected). The whole system should be checked again...AC fittings, pipes, compressor, AC/heater box and its drain tubes etc.
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