An issue has currently arisen and is being investigated by Canon. In the UK a person purchased a Canon 5DIII from a grey market selling out of Hong Kong. When the camera had an operational issue, they contacted the seller and were advised to take their camera to Canon UK for service. This is where the issues began. Canon UK contacted the customer and advised they would not fix the camera as the serial number on the camera was not valid, the serial number plate was counterfeit.
Over the past few days an investigation has started and the source of the issue has been traced back to Canon Hong Kong.
At this time it appears that Canon Hong Kong, without telling the retailers it sold cameras to, has taken refurbished stock or new stock and re-serial numbered them and sold them as new. The retailers (grey marketers in this instance) were completely unaware of this issue.
If you have purchased a grey market camera recently, check the serial number on the camera plate to the serial number available in the EXIF of your camera. If they are different, contact your retailer immediately.
Below are two images of a Canon 5D MkIII. Note how the C and the A of the genuine plate are touching. Note how the serial number on the counterfeit plate has not got the numbers evenly placed and at the same height.
At this time, how extensive this issue is, is not known, but your grey retailer was not aware of this issue, but they should be able to advise you of what to do, now this has become apparent.
NOTE : These are not my photos, but placed here to show you the issue