Humans, by nature, are rather poor monitors.
I am absolutely not sure about that. At the times when humans still had to hunt for their dinner, and had to take care not to become some raptors snack either, monitoring was an essential part of surviving. Humans are quite good in noticing minor changes within their field of view, or noise, smell, vibration etc. unless they concentrate on something different at that time, then they even miss the elephant in the room (and supposedly men are worse than women in that respect).
I believe that humans are very good monitors, if they want to, know what to monitor and are not distracted.
The tricky part however is to notice whether somebody else is just relaxing, or monitoring his environment. That makes the job for FAA inspectors quite tricky ideed. It only is easy if there is something to notice, and the person supposed to be monitoring is not noticing it. As long as everything runs perfect, you can not tell the difference.