Someone's been sleeping in Jennifer Lopez's pool house, and it ain't Goldilocks -- cops arrested an alleged stalker who somehow managed to live in the pool house of her massive Hamptons mansion ... for several days ... without getting noticed. According to Southampton police, 49-year-old John M. Dubis basically moved into J.Lo's $10 million pad sometime in early August. He reportedly walked around the estate freely, hanging out openly in view of neighbors... and even posted Facebook photos of himself all over the grounds -- before police finally busted him on Aug. 8. Jennifer was not home, but the shocking part is security guards were patrolling the property. Of course, you have to remember the 8,500 sq. foot home sits on 3 acres -- and it's a decent hike out to the pool house from the main quarters.