One day I looked on the 'Levels' section of the 'Frontend' directory of the game. In that directory there are thumbnails of almost all the maps. At that time I noticed there are thumbnails of few levels that don't exist in the final game. That means those maps are either deleted scenes or super-secret maps. Any idea?

They are probably maps which were in the game at development stages, then removed for whatever reason, but not totally removed.

The same goes for a few of the things in the weapons folder.