Actually, there's one clear example i can think of to illustrate the extent he went to, and to explain how creepy this is.
What if a person went to the lengths to find out and record what kind of pornography you might like?
What of it? I guess it's dependent on the person and how much that embarrasses you.
Looking at the situation, especially when mental illness is a contributing factor, this kind of behavior is not uncommon. It comes from a desire to control - he wants to be sure you're on the straight and narrow with him, and not say, a government agent infiltrating his group. It sounds silly, but years ago I knew a guy who operated kinda like this where I worked. He was big into unions, and that big business are the ultimate evil. I'm a survivalist junkie, so me and him got along pretty good.
We had been working together for about six months when me and my wife moved out of our old apartment and into a full house. We hadn't moved anything over yet(services, other then power), but he still showed up on our doorstep at three in the morning, because he was on his way home and the police were outside his house(turned out to be another matter with the guy living downstairs from him).
People are also arrested all the time for things that aren't big, for example: protesting! The moment a protest is considered a disruption of the peace, the police will fall in and start trying to break it up. If there's one thing I know about the type, most of these guys have a big mouth when it comes to the cops, and all it takes is a odd word. Mind you, this also depends on the offense.
What you need to do is establish boundaries. Sounds silly, but let him know it's not acceptable to contact you at home. It's what I did, and we're still friends. He may get offended; people often initially do. After that initial shock through, usually the "too far" realization kicks in. Good luck, and have patience.