All of the answers are correct in some regards. Undigested food can pass through the colon as some things can not be broken down. These foods are called unsoluable fiber, which simply means can't be broken down.
Fiber is what passes through the colon and traps and removes waste. It also bulks the stool because it is not digested.
On the other hand...if a person sees "nuts", "seeds", "rice", "corn" or "rolled tomato skins" and they KNOW they have not eaten these foods. then it might be a sign of parasites.