A team of 3 climbers were top roping a route at the end of the day as the last climb before dusk fell. As the climber ascended, the two climbers at the base chatted. When the first ascender reached the top of the route, visible on the skyline to the two climbers at the base, including the belayer, he gave a 'thumbs-up' signal. The belayer interpreted this as 'safe' and took the climber off belay. The climber then shouted down 'Why have you taken me off belay'. The belayer put him back on belay and lowered him to the ground.