Some systems, like lawful intercept, are designed to be hidden even from telco network management systems. The LI console that set up a wire tap might log activity at that particular console at that particular law-enforcement agency. But if you don't know where to look exactly, good luck.

This is why the Chinese picked lawful intercept as a hacking target for the salt typhoon exploit. It's almost impossible to know whether that exploit is continuing or when exactly it began.