Although i do like chubby's ideas, i'm with the walk outside and have a chat crowd. Simply approach him and ask if there's anything you can help him with. Gives you a good excuse to get close enough to the car to look inside and see if you're dealing with a detective, private I or what. No real sense in being shy about it. So long as you are amicable, you are well within your rights to ask if he needs assistance and go on from there to try and find out why he's been there on multiple occasions (while making your own conclusions via observation).