She started doing it at the shelter?
Maybe she's stressed. You might want to try Feliway Comfort Zone plug ins or Spirit Essences.
Play would also help. If you can't get her interested in the toys you have, try different ones. Da Bird or a lazer light are usually big hits.
Also, you may want to make a place for her to hide in the main rooms, like a cat tent or condo (those ones that are like a cylinder with an opening). That's harder to do I have one that likes to hide and it's hard to find the right kind of hiding hole, no matter what I provide she prefers under the bed.
Also, you may want to have her checked out. Cats hide when they are in pain, even if they don't appear to be (as I was oh so recently reminded by my Kitty. Where hiding = ear infection).