In the first level of Doom 3 called Mars City, follow the path to Marine Headquarters and you will come to a bathroom. On the right side there is a door to the bathroom. Inside there are 3 toilets. You need a flashlight so use the "Give All" cheat because you don't get a flashlight that early in the game. Go to the middle toilet, and there you will see unflushed poo. You have to use your flashlight to see it.