You cannot find the security office on the 2nd floor of the airline tycoon evolution game

It's in the middle of the floor, right above the lobby and escalators. Between the advertising office and the airport manager's office.

The security office is partially invisible--you can only see a slit, which opens into a door when you walk through. To find the door, drag the mouse across the screen (in the area specified--the second floor, right above the lobby). The mouse icon turns green when you are over the invisible door.