My Son's troop is extremely meticulous about this.
Yep. Been there, done that. On a three-leader summer camp excursion, I've also awakened my son in the middle of the night so that he could walk with me to take a Scout to the infirmary, leaving two leaders (awake) in camp; my son was permitted to return to the campsite with me, one-on-one.
There are circumstances when one-on-one contact is necessary, such as a two-Scout, two-adult trip where a Scout or leader is injured and left behind. I'd send a Scout/leader crew for help.
In this case, if there is a second Scout or a second leader who can stay behind, then he should have done so.
I've prattled too long about the Guide to Safe Scouting. What's really important here is finding a lost Scout.
I'm with you on the two-deep leadership issue.