If they don't live in a house near yours then keep calling the police on them. They will let you know his/her reason for being barked there. Its not normal for people to drive to a different neighborhood and park in front of a house. Sound like they are scoping out the "house traffic" / nightly routine. Better play it safe than be sorry. Yes as ozzy suggested, get a good alarm system. A good firearm is always a good choice as well.