Why are my cats peeing and pooping everywhere in the house out side of their litter box?

We have had both of our cats (boy and girl) since they were 7 weeks old, they are 3 yrs old now. They always used their let box, but over the last few months we are finding doings and puddles around the house. My dad even caught the boy cat spraying the door panel once. We think its the boy more than the girl but with two cats its hard to give blame unless you catch them in the act. They go out side a few times a week, they have been allowed to play in the back yard since they were 1 yrs old. Why are they doing this and how do we stop it because my parents call me all the time saying that the cats are close to becoming outside cats. I really don't want that since they lived their whole life indoors. So I need help please.