Are those your only return vents? If not, what do the others look like? Kitchens produce a lot of particulate matter that may be responsible for this coloring. What type facing do you have on your fireplace? Can you detect "soot" here on it or the mantle above? What about the ceiling? Not sure how far it is from the fireplace to the filters but soot should be noted elsewhere if this in what it is.

Just my thoughts.

jrb451, We do have other returns that were ok. The soot is coming from the gas logs for sure and not the kitchen. The ceiling is a full story above the fireplace and better not have soot on it as the whole interior was repainted two years ago. This did not happen last year and has just started since servicing a month ago.