The easiest way is probably to create a bullshit project with a few people. We are only creating a new dashboard for X, this is cost reduction project, etc.
I don't know how Yahoo is organized but if teams works in silos, without any visibility on other teams, it is probably not that hard to introduced changes that are undetected.
In a place where everything is monitored, down to the MAC address of machines and their network traffic, ideally it would be difficult to sneak in a monitor.
Access to critical data should be similarly protected.
These are relatively tame intrusion detection systems that you would have to make changes to in order to remain undetected. That should be really hard to hide.
I don't know how Yahoo is organized but if teams works in silos, without any visibility on other teams, it is probably not that hard to introduced changes that are undetected.