The guns were used most likely to fight the walkers that had turned from their dead.
Small piece of evidence, the shotgun found at the bottom of the stairs.
No one found out because everyone wanted to avoid Crawford. As seen when Molly attacked Lee and Kenny, also when Lee found Vernon they are afraid of being found.
Most likely only a handful of people lived, I believe even less. They must have thought they were the only ones alive in the city and kept quiet.
Sometimes I feel a bullet would solve all my problems