I have never gotten any (that I knew of) either, but some of them I know what they are. Two are pretty obvious encounters: One is a big tank with missiles scattered around, and the other one is a ladder hole in the middle of the map.
I am not sure about the others, but when you encounter a special encounter, it won't be a combat encounter and you won't encounter any other players, so if you ever get into a mysterious encounter that doesn't have a "You encounter... wastelander" on it, then look around for a bit.