recently read an account by Labatt where they put out a trip line & bouy. Another boat came along after dark and snagged the trip line and started dragging them.
I've read other people thinking the bouy was a mooring bouy and tied to it!
I like the suggestion that you put a piece of poly rope on the anchor but set the rope so it floats 5' to 6' below the surface. If you need to trip your anchor, you can get another line down to it with a boat hook or just swim down to the line. This method keeps the line out of virtually all props and out of the reach of others.