In 2005 I was both making Quake 3 mods and working a QA job.  I decided it was time to build a professional quality demo reel of a single player experience.  Since I was already familiar with previous Id software engines, I decided to use the then recently released Quake 4 to build the level.  In a twist, I built a gameplay experience similar to Metal Gear Solid in which sneaking was preferable to direct attack.  The monsters would change states of alertness depending on where the player was and if the player was seen by the enemy.  This was done entirely using the Quake scripting system.