Your manager left early. Your standards didn’t get the memo.

Doing the job properly once supervision disappears is where work stops being about compliance and starts being about you. Nobody notices at the time, and the habit settles in regardless. Whatever standard you hold while unobserved is your actual standard, because that is the one running by default. Being watched only reveals it.

Finish the last part of the task after everyone has gone home. Do it the same way you would with somebody standing there. That version of you is the one that turns up when it counts.