The office has three 'hidden' rooms that need to be unlocked. Two doors are obvious, the third door is behind two lockers in the room opposite the first door and leads down to a secret trophy room.
To hack the doors, look at the screen as you enter the numbers. If you enter a right number in the right position, the screen will shortly flash. Thus the easiest way to figure out the code is to enter all numbers 4 times and note the positions of the flashes. You may want to get a piece of paper for the door in the room with the dispensers, as it has a code 17 digits long. All door codes in the office are randomized, so looking them up is impossible.
Once you have all the doors open it's a simple matter of cleaning up as you would any level. Make sure to keep the two notes you find, as you will need them for another achievement later on. You have to remove all trash, blood, viscera. This does not include loose objects like chairs and cabinets and does not include the dirty toilet water.
If you can't find the last bit of dirt, check the trophy room. Bits of blood on the ceilings are sometimes hard to spot.