When my tabby cat, Otis, was young, he'd get stuck behind the television console every time we left home. I don't know what he was trying to do, but we'd have to move this big heavy piece of furniture when we got home and he would race to the litter box. Sometimes we'd be gone overnight, or even for two days. He never peed behind there, but always held it until we came home to release him. Twelve hours doesn't sound like much compared to that for a young cat. My old cats couldn't last that long, because they pee so much. As long as you are providing a litter box for her when she sleeps in the garage, I wouldn't worry about it unless she is peeing elsewhere.