Cats are able to track their homes down by scent, much like dogs. Their cat may pee outside, and this is normal. They are marking the area with their scent so they can
a) ward off other felines
b) know that it's theirs
They are also able to remember where they are and how to get places.
I do suggest that your parents keep their cat indoors, but if it does get out, they shouldn't panic. If the cat doesn't know the outside very well, it's unlikely it'll go far, so they should be able to keep an eye on it. In fact, they may want to go out with the cat, on a nice day!