The garage glitch is a pretty old thing, I remember using it in GTA3. That's because it is computationally expensive to clear the screen after every frame drawn. Computer games generally don't bother clearing the screen since new things will always been drawn over it in the next frame. However, it can be noticed when you go outside of the area you're expected to be in, such as below the level, or outside using noclip on whatever game it is. This is because the entire scene isn't being covered by newly drawn material each frame.