Sounds to me like the main pain is inside and no one will open it up to take a look.
I bet you got a system that is not level and no one will tell you that this is causing the unit to rust were the water is pooling and leaking cause the water never reaches the drain.
Most likely its not level cause your foundation shifted or because some one used the incorrect base which would be something like wood that deteriorates over time.
The only other thing I can think of is you have negative pressure and your AHU is sucking the water back into the unit.
If you're too "open" minded, your brains will fall out.
Artificial intelligence is no match for natural stupidity.