I don't really find it odd. Unless I have some of my facts wrong, MB was found pretty far away from the entrance, in a room in the SW corner. We know the first camper arrived at 4:35. We know it rained a lot that morning, so those first campers could have been waiting for a lull.
Then when they did enter the building, which I think I read was no earlier than 4:45, they could have milled around just inside, thinking MB was in the bathroom. And remember, MB was not a church member. Some or maybe even all the campers weren't, either. So probably not a comfort level to just go roaming.
But they knew her vehicle was there, so probably 4:55 they check the bathroom and then start looking further. So the 5 am discovery makes perfect sense to me. JMO