You have to consider how isolated this area is, and how big the towns are.
It is about 25 miles to Hartford, and 10 to Bloomfield. In order to get to either place, you have to go ‘over the mountain’ on either 185 or 44. This is a long drive on two-lane roads in the dark.
Anyone who is in any way thuggish-looking will stick out like a sore thumb in Simsbury. I once saw one black guy walking along 10, but you are highly likely to be seen and noticed. There would be no reason for anyone like that to go there at that time.
The bicycle path behind the shopping/restaurant area is quite well-lit, and cars drive by on Iron Horse Boulevard all the time. Many people would have been leaving restaurants like Plan B and Joe Pizza at that hour, so there should have been considerable traffic.
Something very unusual must have happened. It may have been a planned ambush. If it was a sudden affray, the only thing I can think of is a dispute with a hot-tempered Lyrca-clad bicycle rider over the right of way. He would have made a quick escape on his $2000 bicycle, and washed the blood off the knife in his garage.
I've been through the area, even stopped at a restaurant where you describe. Those restaurants do attract people passing through; it's not a totally isolated town with no strangers. It is shocking, to think she probably would've been safer jogging in the park in the south section of Springfield.
Do those wealthy serve the fries and clean the toilets in utopia?