Prospect Hill is a location mentioned in P.A.M.'s terminal entry 2284 found at The Railroad HQ. The location in the real world is shy north of Cambridge but in-game- I've discovered that the location is home to a small Raider camp and what appears to be a small town. Nearby is Wattz Consumer Electronics.
It seems to be, from what I can tell in the terminal entry, that Patriot, an Institute character who helps out The Railroad, has provided coded holotape messages that provide specific instructions for safe passage from The Institute grounds to a safe drop-off location, in this case Prospect Hill. Not much else is provided about the location in game however a Wikipedia article about Prospect Hill is available. Apparently its a historic hill in Somerville, MA. -- which is part of Charlestown. It has a small public square called Union Square, which was originally called Sand Pit Hill. Other mentioned only locations include The Bolthole, The Beast, The Farm (south of Parson's Insane Asylum) and Randolph, all Railroad safehouses at one point or another.
Do you know any mentioned only locations in Fallout 4 or Fallout 3 that actually appear in the game?
Here's a video I've created showing you where Prospect Hill is in Fallout 4.
