When entering the North Sound via the channel there are 3 red buoys, 2 of them have corresponding green buoys and mark the channel. The 3rd red one marks the extent of the reef when heading towards Leverick Bay and I think that this one is easy/easier to miss. If you don't see it, then a turn directly for Leverick after the second red buoy might lead you onto the reef (if you aren't watching the water color and depth sounder). That might have been what happened in this case.