I've had my d5100 for almost a year and a few weeks ago I noticed that on a few of my photos shot against a blue sky there was a very faint overexposed arc towards the left third of the image (landscape mode). I was able to track it down to only images taken with very fast shutter speeds (>1600) regardless of lens used (in one case, a Sigma 150-500, in the other, the Nikon 10-24mm wide angle). This artifact was in the same place in the images regardless of the location of the light source. I've put some sample images here

http://www.billkraus.com/Other/Nikon-D5100-problem/27396694_Dn3rcV

I sent my camera into Nikon two weeks ago (just a month shy of the warranty running out), described the problem and pointed them to the example photos. I received it back today - but the repair notice indicates 'no problem found'. Unfortunately, the weather here is such that I won't be able to take test shots for a few days, but I was wondering if anyone has seen this before, as I am worried that they didn't really investigate the issue. Could it have been something inadvertently fixed via the complimentary 'check & clean' (fyi - my sensor has been cleaned before this)?

UPDATE: Its still there. Used yet another lens (Nikon 55-300) and took a few shots on a very overcast day - sure enough, the arc was there at fast shutter speeds. Contacted Nikon and they asked me to send them some sample NEFs. This artifact can't be normal.