@strypey there is always a lot we sysadmins don't know we don't know!
A mantra of the terrain really. Adversaries, including automated, make a game of this.
I myself am studying every day, in some shape or form. The learning never stops, and this is especially the case as regards server hardening/security.
Several of my own students over the years have been senior sysadmins (20+ yrs exp) and said they've grown much through the training.
There's always something!