This is what RJ Roman's walkthrough says:
If you are playing in the ENT mode you simply look for Dag Horton’s office door and enter. (Remember the cufflink with the initial DH) However, if you are playing in the GP mode when you enter the hallway turn left and rush into the storage room. While in the room get the box of soap from the shelf and the mop.
Timing is of the essence here, so, this might be a good place to save your game. You have a security guard to contend with in this area in the GP mode. Wait until the security guard passes and you hear the door shut. Rush straight down the hall to where you see the bucket and mop the floor. There will be a cinematic scene of Tex mopping the floor. When it’s over, open the door beside the bucket to trigger the alarm, then rush back into the storage room and close the door behind you. When the guard makes his rounds again he’ll slip, fall and be knocked out unconscious. In the next cinematic scene Tex will drag his body in the closet and lock him in with the mop handle. Now, you can enter Dag Horton’s office.
Here's another - Check Day 3 at UHS Hints.