I just noticed my M8 developed this problem a couple of days ago, whilst reviewing images on the screen. More obvious on bright scenes. The camera's been used sparingly over the last 2 months or so and have been in a camera bag most of the time, in cool conditions. Yesterday, I sent it back to my Leica dealer and the immediately took it in to be sent back to Leica UK.
I guess this is due to poor supplier of the glass or installation and is a known fault? Have never come across this problem having owned so many digital cameras over the years.