http://www.twitch.tv/marinersfanstev/c/3579179

https://www.youtube.com/watch?v=Pe5atj2pz2o

https://www.youtube.com/watch?v=k-KiJLopU0A

The first step is to stealth Firestarter day 1 but burn the weapons (and burn bagged bodies?). doing this would cause a FBI helicopter to appear. If you or a teammate quickly kill the FBI agents that come out of the helicopter you'll still be in stealth mode EDIT: An extra bag also appears after killing the FBI agents.
2 The second step is to be in stealth mode in day 2 and have all 4 teammates to stand in all 4 FBI seals. After doing this some smoke appears.

https://www.youtube.com/watch?v=nwIG3HifLcU&index=4&list=PLPc9Dip-LwV3Uq4-tVBB0e9DiN0M1CZMC