Sometimes if there is an underlying health issue, cats will go outside of their litter boxes. Has she been checked out thoroughly by a vet?
If health issues have been ruled out, has her brand of litter changed? Sometimes that will cause cats to not use their litter boxes - if they don't like the new litter. Also, if the box isn't kept clean enough (to the standard of the cat!) - that could cause her to go elsewhere.
Before she became part "outdoor" cat did she pee around the house, or did this start around the time she started going outside to play?
Does your dad have any other cats/pets? Any new additions recently?
When she does pee around the house, is there a specific area she goes to most often? If she only has one litter box, perhaps try adding another one - and if there is a specific area she favors, place the new litter box in that area. Or move an existing litter box to that area.