In Basso Job : check Mate side mission, you are tasked to steal Basso's chess piece. You need to break into Archie Maxwell house and steal his stash to find this chess piece.
Thief side mission- Check Mate Walkthrough
Travel to the location and enter Archie Maxwell's House through the window. Loot the place and don't forget to grab Archie Maxwell's Diary from the table, and once done go down the stairs slowly.
There is a guy standing by the table, while crouching sneak behind him and knock him out. Loot the room and locate a panel on the wall. Press the button (nothing happen), now pick the lock of the locked door to get to the next area.
Go down the stairs and open the door to the adjust room on the right. Unscrew the vent and crawl through it to get to the other side. Just ahead press the button on the wall (nothing seems to happen either), then back through the vent and then turn to the left toward the objective marker.
Use Focus to locate a control box, hack it and then go to the wall safe and enter the code 7, 7 6. Grab the Basso's Horsie to complete the mission