CS 2 has been developed for at least three years. This was announced by game art designer Robert Briscoe. Maybe the game started earlier because the designer only talked about his work experience. Valve Studios did not officially announce when they first thought of porting a shooter game to the Source 2 engine.
Previously, references to new items were found in game files. Perhaps Valve will add a twin blade sword, a Kukri knife, a pipe, a new box containing agents, and other things to the game. Some of these may only appear in Danger Zone mode.