At the fastest speed, the radar can sometimes "miss" the wisp warning obelisk. So the entity number goes down one. Then the next time it sees it again, so the number goes up and the alarm sounds. This happens every 10 seconds. Downgrade the radar speed 3 or 4 times will make this go away.
My request/suggestion: Remove the obelisk after the Pyramid event. There is no need for it to stay after that.