I don't see any flash from a cold boot (say from overnight) but if you have had the keyboard powered and switch off for 30 secs and back on again there is a very faint flash in part of the display (almost not noticeable), before the normal blackness then grey backlight, then intro scrolls etc, and is probably due to some charge remaining in the capacitors and power supply not having leaked away yet. If it's happening from a cold boot I would guess the voltage is just surging through for an instant before the regulators have had a chance to gain control and bring the display up softly after the microprocessor has gone through its' boot routine.