I remember when we bought a BVW-300 in 1990 and suddenly found dead pixels after a cross-country flight. We went to the Sony facility in New Jersey (we were shooting nearby) and this was before they had developed any policies about image block replacements nor had they developed the de-tuning abilities to map out dead pixels. At the time, their best guess was radiation, altitude, heat.....in other words, they had little idea. It could have been anything - even just plain old use. Those pixels are tricky. In general, they can come and go. I wouldn't particularly worry about it, as long as there is a way to map them out temporarily (Sony's black balance can do that).