(Metal Gear head canons (both mine and the Internet's) and Studio Ghibli are to blame for this. My Metal Gear knowledge begins and ends with Snake Eater, so please tell me about any mistakes I make with Guns of the Patriots events. )
One
Sunny first noticed the mansion on the day she moved in to her new home. She was sitting in a taxi with her Uncle Hal when she noticed they had stopped next to an old, abandoned mansion. Sunny had never been to this town before (or anywhere else other than Nomad, for that matter), so she was surprised when she saw the old building and felt how familiar it was.
Before she could examine it further, the lights changed, and the taxi drove off. Sunny spent the next few months helping move in and visiting her Uncle Dave in the hospital. She would pass by the mansion every week, and would stop to look at it when she could, but there was never any chance for her to explore beyond the rusted fence.
Until her Uncle Dave died.