Piano Forty SevenEdit
To complete this challenge, you need to knocn someone out with the piano.
Head to the northern part of the second floor. In the rightmost office (east of the room where you picked up the keys to the speedboat) you'll find a piano.
Activate the pole holding up the piano, and hide. An auction staff member will come to investigate the piano from the other room.
Wait for the auction staff member to fix the piano, and to investigate it more closely. Once they do, approach the piano and move the pole again.