I have the dark staining of drywall in an apartment that we renovated 8 years ago. We have Mitsubishi Heat Pump and the cathedral ceilings and walls were done with flash and batt method. It is in climate zone 5 and I put an inch and half closed cell foam with 4 inches of rock wool insulation. The tenants that have been in there for 3 years noticed these stains getting worse this past winter. They burn candles in the apartment so we are thinking this has to be one of the issues.
I just had the HVAC contractor clean out the fan turbine in the wall unit and we had to clean off the plastic cover. It had quite a good amount of soot covering it. If I had lived there then I would have noticed much sooner. They even noticed the drapes to get very dirty and any other plastic laying around on the kitchen counters.
Could this be due to the thermal bridging of the roof rafters?? I have seen the darkened lines and screw marks in lots of other buildings throughout my years of work. 
Obviously I need to clean and prime with stain killing primer now to remove these stains however I am not sure if there could be an issue with this HVAC unit. 
Any input would be appreciated? This is my scare of making buildings tight. This apartment is only 900 SF on the second floor of a small mixed use building and we have electric heat matt for the tile floor as extra heating.