If he is there, it is not about a grow op... Undercover 5-0 or a CI would be doing that. 99% sure... Unless it is that someone paid him to get dirt on someone, possibly a spouse, etc.
See which area he is watching... Often times its the backward area they watch... Head rests block view of occupants from the rear, and they watch mirrors, not directly. Or through the back of a tinted window. If the front of his car is facing toward your place, chances are its a house/people that live someplace beyond the back of his vehicle, a few houses down typically. If you have double lane parking on your street, he would more than likely be on the opposite side of the street from the house he is watching...
Just by seeing how they are parked, and thinking like a CI would think, you will be able to narrow the window of what you think he is up to. If you can do it with no one watching, see if you can figure out the geometry of his rear and side view mirrors, they will more than likely all be pointed towards the same object, if they are, that is the target area.
Just for the simple fact that he told you he was a PI, I highly doubt its you or your family hes watching. He would have more than likely gave you a quick and easy lie. A quick perfect one would be that he was private security, and performing security watch for a person on the block that might be in danger.