Did an inspection the other day and when looking at the evaporator or inside condenser as this home had a heatpump, I noticed that there was a duct that appeared to be going into the garage. I made a mental note to look at it later when I went down from the attic and back to the garage automatically assuming it was a register. It had some sort of controlled baffle in it before going into the air handler. What I found was in the garage it was sheetrocked over and actually vented outside. Then it dawned on me that it was connected to the return side of the air handler before the filter.
So apparently some sort of fresh air inlet? Not something I am familiar with. :-k