There is probably always some mucous, just some times it's not as visible as others.Mucous's main purpose is to protect delicate tissues that are exposed to outside influences. In your nose it keeps it from drying out, and when you get a nasty in there you coat it with mucous and sneeze or blow it out. In the digestive tract it is a lubricant to help the stool slide along, and if your GI tract thinks anything nasty is around it coats it with slime to isolate it until you poop it out. K.