Cave evidence - the fact that stolen sunglasses were found there is enough to convince me that Hart did stay at those caves. Add in the fact that at least 3 people had visited and/or stayed with Hart at the cave/cellar camp during previous jail escapes. Hart was a prison escapee for 3 years at the time of the GS murders. While he stayed with some friends/family in the area during that time, he also spent time in the caves, usually when LE was searching for him. One of the caves was located across from his mother's home where he could easily see the comings and goings of the local police and sheriff.
Hart was stuck hiding out in the general area for over 3 years. He was isolated, not like he could have an active social life. No dating, etc. It's not surprising he would be creeping around Camp Scott when scouts and counselors were there. Perhaps he escalated from voyerism (recall the incident the week before camp began when CarlaW reported hearing a man creeping around the staff building when she was sleeping there?). Peeking in windows and tents wasn't enough and he wanted to do more. I'd be willing to bet he was also engaged in voyeurism at homes in the Locust Grove area, too.
JMO, he needed an accomplice. Spending so much time hanging out in the area, he probably had visits from some of the teens and young men who lived nearby. Some were relatives. There were locals who reported seeing him out roaming around, hunting and fishing with young men. They could probably go hang out with him to drink, smoke MJ, do drugs, listen to his stories about prison life, etc. The men his own age were either in prison, jail or married with jobs and families, so he probably had to settle for hanging out with a younger group. He probably spent some time talking about his previous crimes, telling them how to abduct and rape women, break into people's houses while they're sleeping, etc. All those stories probably turned into a plot to kidnap and rape some girls from the local resource - Camp Scott.
The only question left, IMO, is which members of his young crew went with him that night?