When I was attempting under the radar, I flew under every bridge, as I did so, a notice appeared at the top and said, you have so many bridges left to fly under for the acheivement. On the map there were "trophies", but as I got to the last bridge , the trophies disappeared and I couldn't get the acheivement for under the radar. I didn't use cheats so I don't understand why I can't get it? Can anyone explain? It's forty g's!