As Dennis Nedry prepared to cover his tracks to hide his theft of the embryos, he jammed the phones as well. Knowing that the staff back in the control room might attempt to call the mainland to help fix the problem with the computers, he inserted a lockout into the program code. This command was erased when John Arnold erased the White Rabbit Object trapdoor which had erased part of the program listings. However it was still lodged in the computer’s memory. The command to jam the phones was RAM resident, meaning that it wouldn’t show up in the code either. Even with a RAM dump, it would’ve been impossible to locate the code without knowledge of what the title of it was.
Jurassic Park by Michael Crichton page 255 (First Edition, Thirty Fourth printing)