I agree. I don't think BG was walking back and forth along the trails in plain sight, since he probably wanted to limit his exposure to potential witnesses. I think he found a convenient place to park, maybe even secluded from view, from which he could get to the bridge. He decided to act when he had the girls cornered at the end of the bridge and he could see no one was about to cross the bridge at the other end. He got them "down the hill" as quickly as possible, so they'd be out of sight. I think he killed the girls, got to his car, and left the area before anyone knew something was wrong.