I have read in a walkthrough that in this mission, there is some sort of a security office behind the lobby reception counter (in the area behind the only employees' door you can't open with the master key, opposite the east elevator on the ground floor). He said that the desk clerk will let you in there twice if you're wearing a policeman's or bellhop's uniform. However, I could not get this to work.
Since this mission was remade for Hitman Contracts, I assumed that this particular guy just confused the two games, and that this is only possible in the remade version. But then I played around a bit with the cheat codes/debug mode, and I saw that there are actually fully furnished rooms in that area, with desks, chairs etc., and some guards. The door behind the receptionist and the locked-off employees-only door seem to be the only entrances to that area. I couldn't open either, and couldn't find any other way.
Are these just game-internal rooms to store reinforcements to let loose on the player at some point (although why would they be furnished?), or is there actually a way to get in?