I have noticed an increasing number of these. One way along a route the white marker buoy is there but on a return - it's gone. Even if the return is just a few minutes later. Weird. Sometimes, there's a pot marker dead ahead - so i make the course correction, but then checking again and the thing seems to gone. One one occassion however, i saw a marker bouy dead ahead and instead of making the correction i kept watching it and before we got there it FLEW OFF! So perhaps the answer is to not flinch but just to runnem down.