I was just there like a month ago and cops were coming through non-stop. I got yelled at for being on one of the basketball courts and pointed out that the sign said "stay on the pavement" and the court and sidewalk leading to it was paved. But went right back to the road of course.
Then I ran into some other people taking pictures who were visiting from PA and they said they got yelled at too because someone had just set a fire in the big white "elevator" building the night before. Not sure how much damage was done as I didn't go in this time with the heavy police presence. Saw a cop (or maybe just security) car across the street just sitting there so I walked the loop and just kept going.
Normally you can do whatever you want and just have to be aware that every hour or so on the weekends the cops will take a ride through making sure you aren't doing anything bad. If you're in the main part, they just make sure you aren't peeking out the windows of any building basically. But if you park on the other side by the golf course, they've chirped their sirens a few times to see if anybody comes out of the woods as they seem to be really concerned about the power plant area.
I've heard rumors of them checking people's cameras before to see if they took pictures inside the structures...so it's not a bad idea to bring 2 SD cards and swap them out when you enter the structures. If they hassle you, the card you have in your camera just has external shots and that should put them at ease