Early '70's. Yes on hitching in the Five Colleges area in the 1970's. Yes on a Five College bus between the campuses. But you couldn't travel between Mount Holyoke and Smith: you had to go over to Amherst first. There was virtually no public transportation between this area (especially South Hadley, which I'm more familiar with) and Springfield, Holyoke, well, anything really. Not many young people had cars. Thus, the need to hitch. People worried about hitching, but that became more of an issue in later years.
IRC, Granby was on the Five College bus route, but you couldn't get off. Parts of Granby are on Route 116?
Could the UID have been on a bicycle? Is the trailhead/entry to the woods visible from the road? She could have run in there to get away from someone and was unsuccessful? She jumped out of a car?
The magnet may have been used to defeat security devices in retail shops, if they used magnets in those days. The shirt (or other item) could have been hocked? Maybe the magnet was for something similar, but not exactly that? Stealing coins? Jewelry?