During the mission where you infiltrate the White Box company, after killing the guard by the front car lot, infiltrate the building. When you first see the room, you can either climb up the pipe and throw an object (frag grenade, proximity mine, remote mine, sticky cam, or EMP grenade) into the room. Even under the Realistic difficulty setting, the most the AI can do is shout and raise their guns, provided you have upgraded each gadget to the maximum. This room provides many ways to get and complete any challenge, except for most of the Splinter Cell challenge group. Also, this can be repeated by reverting to last checkpoint an infinite amount of times.
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), kill all the guards. Walk down the stairs to the chandelier. There are some boxes directly under the stairs and a slightly opened body bag leaning against them. If you look inside the body bag you will see Archer. There is another body bag in the center of the room, but this one is completely closed. This is assumed to be the dead body of Kestrel.