I've been a fan of the Halloween series since the late 1990's but my interest in the filming locations only began at the end of 2014. At some point after watching Halloween III I became interested in the location of the hospital, but after searching through IMDb, forums, location websites, and viewing Sean Clark's 'Horror's Hallowed Grounds' it became clear that the location was unknown. After some thorough research and an exhaustive search I managed to find the location, even though it had been demolished.
Eventually I posted the details to the filming locations thread on the Official Halloween Message Board, where I then noticed the huge list of unknown locations from the entire series, including some major locations that I had expected to be known. Over the next couple of years and with a massive amount of research I managed to find nearly every unknown location on that list (from Halloween to Halloween: Resurrection).