That means she does go out and it's probably only through sheer luck she hasn't fallen pregnant yet. When she comes into season, she'll be driving all of the boy cats in the neighbourhood absolutely bonkers. It only takes the time from the house to the end of the driveway for her to get pregnant!!
Don't spill a load of crap about "not being able to afford to have her spayed". Do you know how many unwanted cats and kittens are abandoned each year? You're irresponsible and my comment about you (or whoever is responsible for the cat) being an idiot still stands.