It's called the night shift.
Carey got to be in charge completely when B'Elanna was asleep.
What do you think happened?
Two Lieutenants working side by side during the day shift, and then when they both go to sleep the Ensigns run amok?
Of course, that means that Lieutenant Carey had to report to "Captain Kim" whenever shit got real.
If Carey was awake and on duty during the nightshift, what the hell is he doing hiding down in Engineering when he should be on the bridge?