When Frank Fantasia was vacationing with his family in Nantucket, his son made an observation: if some of the large summer mansions there were occupied only a few weeks a year, there must be someone to keep an eye on them the rest of the time. The thought quickly evolved into a business idea for Fantasia, who had recently retired after a 30-year career in high tech.So Fantasia, a long-time Winchester resident with entrepreneurial streak, decided to give his own business a try.