Even if the captain leaves the bridge he should still be able to maintain awareness if need be.
In this case the captain left the bridge while the ship was on an unsafe heading, if you roughly know the time of the turn, keep an eye on the time, if you can’t feel the ship turning on time something is wrong. Plus you can take a glance out the window to see if Bligh Reef light is where it ought to be.
In that situation captain should feel “chronic unease”.
SOP when a green mate is on watch, that’s why they are on the 8x12.