Its an interesting question. If someone unauthorized was on your network exfiltrating data how would you know?
So even if it's someone with valid access, it would be investigated immediately.
> Accenture told Marriott's IT staff that one of their security products, a database monitoring system called IBM Guardium, had detected an anomaly on the Starwood guest reservation database
https://www.zdnet.com/article/marriott-ceo-shares-post-morte...
Hiding on a box is easy. Hiding on the wire is hard.