An Internet Meme, Trollface is a black and white drawing of a face with a large mischievous grin that is meant to convey the expression someone makes while trolling a board. The use of the Trollface image in a forum message is generally used to indicate that you believe that someone has been fooled or intentionally angered and is reacting to a Troll.
The face commonly appears in rage comics indicating that the character is being mischievous in some way.
In the level Pillar of Autumn, after you receive the pistol from the Captain, look at the bulletin board on the way out of the bridge and you will see, in addition to the Trollface. This video is from our unofficial game guide. You can view the full guide here