But that's not what the engineer allegedly said. He allegedly stated that once a circuit is installed, its voltage should not be changed. It would be incorrect in stating it as an absolute, if he did in fact do that. However, as a general guideline, it is a very valid philosophy.
If you cannot trace 100% of that circuit, then you cannot be 100% positive that there are no taps. For example, there could have been an exposed junction box on the original install, but then someone installed a wall partition around that box and did not leave the box exposed. What you may assume is just a simple wall penetration could in fact be a buried junction box with a tap. You viewed 99.9% of the raceway, but that 4.5-inches that you didn't view is enough to kill someone.