What is cause of dark smudge line on UN40D6000 LED HDTV

Purchased UN40D6000 edge-lit LED HDTV earlier this year. Small issue noticed - not sure if it is getting worse or if my eye was just being drawn more to it. -- At mid-screen (L to R), about 4" from top of screen is what looks almost like a "smudge" line about 5" long and 1/2" high. Visible on screen when a uniform bright picture is in that area. The pixels are working and picture is visible but it looks like the lighting is a bit reduced in this area. Thought at first was dirt on screen, but no luck. Unit is under warranty. Samsung was very quick to provide support. Replacement panel installed - same thing. Then another panel installed, with same result again. This has baffled the repair man and me. We decided it may be that all of them have this problem and we'd try to just ignore it (it is not usually visible - and I am trying not to have my eye drawn to it), but I am at odds to understand the cause. Three panels with the same issue seems a bit unusual. All other parts of the TV are just fine. Issue is on all inputs. Rubbing the panel in that area did not work to correct it. Never saw this on any other Samsung (or other manufacturer) and I went to the store to check with the current models - no problems there either.
Wondering if anyone has had this problem (or knows of this type of problem) and has determined the cause and possibly corrected it.
Many Thanks,