BBM
I camp a lot. Campgrounds aren't enormous, and there are limited places you would think to look for a person within a campground -- you've typically got one or more bathrooms, maybe a shower facility, a place to bring garbage and recycling, sometimes a shack for boating and lifeguard gear, a building to house equipment like lawn mowers, chainsaws, etc., maybe a common building for people to congregate and play games or a pavilion for picnicking, and the caretaker's quarters. It isn't illogical to think that if a member of your party isn't in the tent or your vehicle, then they're probably in the bathroom or shower. Almost every time I've camped within a formal campground I've experienced someone usually bellowing into the bathroom "Hey So-and-So, are you in there?" Last year I even inadvertently freaked a woman out when I yelled back that I was the only one in there, not realizing her young child was quietly standing at the sink waiting for me to finish peeing so I could turn on the water. Knocking on a stall door doesn't seem weird to me at all, nor does choosing that as a place to look for someone.
As soon as I reach any campsite, the first thing I do is lock my phone in the glovebox. Not everyone carries a phone when they camp.