The rule that a T. rex cannot see you if you stand still is straight fiction Michael Crichton invented; the research it is attributed to does not exist.

So, I think it's just straight fiction. It works for the book and it works for the movie, but it's... as far as I can tell, it's straight fiction, and Crichton just made it up. If it's buried in some bit of literature, he's done better finding it than I have, and I've had a really good look, and I know how to look.