During the level where you have to find a member of the inner circle in a fancy apartment complex, go to the part where two cleaners are outside the window on the lift. Kill them both and jump down from ledge to ledge until you reach the bottom. Run along the edge of the building until you hear a shotgun blast from the inside and see someone flying out of a window. Kill the cleaner in his apartment but do not go in. Continue along the buildings edge for slightly longer and you will find an open window. Inside, is a memorial for someone.