Suspicious, Indeed.... but as said by others, he could have been doing any number of legal things. But, I would have surely gotten the make and model of the car, and a description of the fella, as well as, of course, the plate numbers. But yes I do not think that would qualify as an emergency like
coyotehitman said, or anything worth calling 911 over, but yes I suppose where you live would factor into such a decision. I live on a dirt road, mostly ranchland with the smallest lots available being 3.3 acres and most people have 6.6 acres, two lots, so we have plenty of idiot kids who park at the end of the road to do some underage drinking or smoke cigarettes and the like while acting disrespectful(littering etc) I think my paranoid neighbor has called the cops on them enough that this area has gotten a reputation as 'a bad place to party maan...' When I see a car full of kids parked like that near my property, especially late at night if they are being loud, I will walk out and confront them, (not carrying a shotgun or anything heh, at least nothing visible) but just me walking towards their car is usually enough for them to get scared and drive off, if they stay I just politely try to talk to them and ask them to leave and tell them that I have alot of neighbors around who are much less friendly than I am, and wouldn't take too kindly to such behavior
Most cities do have a non-emergency line you could call if it concerned you enough, but I would just sit on the info, and if you find out in a few days that something happened up the street, you might have some good info for the local LEOs.