I've seen them, they are usually accompanied by a graphic depicting a 3-point left turn using pedestrian facilities. Usually at large, busy intersections with fast moving traffic where local bikelane dispensing commitees think people might be at risk attempting lefts as vehicles. Bloor & Sherbourne here in Toronto for example, though that intersection has a better sign than the one you describe. Without the graphic it would make less sense, with graphic it still makes no sense as it wouldn't be kosher to turn left from a bike lane in any circumstance I'm aware of. I guess it could be for right turns if you come to a bike lane street where car traffic is restricted from making right turns. In TO bicycles are excepted from restrictions against turning right at many intersections (Bay & Bloor) but not all (Queen & Yonge)