During the "Kobin's Mansion" level in the single player campaign, you can see the corpse of Archer from the multiplayer campaign. In the second room you enter (with the chandelier), eliminate all the guards and walk down the stairs to the chandelier. There are some boxes directly under the stairs and a slightly opened body bag leaning against them. Look inside the body bag to see Archer. There is another body bag in the middle of the room, but this one is completely closed and assumed to be the dead body of Kestrel.