Are you positive that you have never opened your server to other people? It may be very possible that somebody built that, because I doubt that would appear out of nowhere.
Glass is not a naturally occurring block, so I really am clueless as to how this happened without somebody doing this. Notice, however, that there is a cobblestone block on to off the glass - this would have to mean that somebody has been here before.
It could be possible that a glitch arose where part of the data from your structure ended up copied in another place (did you download any maps/packs while doing the mountain project of yours?) but that is unlikely because all of the maps are generated the first time you enter the game, and any changes from there are made by the player. So, it would either have to be faulty data, other players being on your server, or the occasional Herobrine.
