What is that smell?

Ok, I'm sure I'm going to get a bunch of smartass answers here. But let me set the stage. We have a bedroom in our house that is basically a spare bedroom. It is our teenage daughter's room who does not live at home as she is away at college. Over the summer (while she did NOT live here either) we started to smell what smelled like cigarettes. Our daughter is allergic to cigarette smoke so we are 100% sure it is not something she brought home. Again, she didn't live here all summer as she was working in another city. When she finally did come home for less than a week before going to back to school, she pretty much could not sleep in her room as it made her sick to be in there with whatever is going on in there. She did all of her laundry prior to going to school and when she got there and unpacked she got hit with the smell again… So she had to wash all her clothes and my wife spent the last week washing and cleaning everything else in the closet as it too smelled like cigarettes…

Right above her room in the attic is one of our HVAC systems. So I go into the attic and the plenum is full of condensation. As in it was pooling at the top of the plenum and then dripping down the sides onto the plywood and then probably going into the wall cavity which has insulation in it. So get the HVAC company out and they are stumped as they have never seen this before and don't really know what to do and are throwing all kinds of BS out there on what the cause could be, mind you less than 5 feet away is another unit, EXACTLY the same size and it is NOT doing it… We then start to think maybe it is mold and the smell is ONLY confined to her bathroom and her bedroom (stronger in the bathroom/closet). Have a company come out and take some air samples and it comes back NO MOLD…

Short of ripping open the wall in her bathroom/closet area, has anyone ever experienced this sort of smell before??? Any ideas what it may be and how to get rid of it? I'd greatly appreciate any help that can be provided as we have to keep her door closed and need to figure it out sooner rather than later as it is also making my wife sick…