It looks like what happened with the security footage of the lab door the night of Niko's death was this: The original footage was recorded in Security, with a second copy secretly going to the computer in Niko's study. As soon as Victor left the lab, he edited the copy in Security (which he thought was the only copy) to take himself out. When Gray saw that footage, he did the same thing -- edited himself out of it before handing it over to the police. But he kept what he thought was the original footage and left himself a note to remind himself of the number in his "safe place" (the cubbyhole behind the coat hooks).

What a mess. I wonder how long it would have taken the police to discover that the video they had was actually edited twice before it got to them?