Yeah, most of these 'fell off of a truck' somewhere along the lines. I also know people that would dumpster dive at the local NSS office and cobble together full prints from non-major studio releases. NSS would pull one reel from each print and throw them away separately, making it very difficult to get a full print.
Also, I have a few slightly damaged replacement reels that were never picked up by the studios. Most felt it not worth the bother.
A friend of mine bought, and then re-sold, well over 100 35mm films, including the Star Wars trilogy, most of the Treks, The Princess Bride, Back to the Future, The Terminator, Aliens, Tron, The Dark Knight, Point Break, Titanic, The Last Star Fighter, Night of the Comet, The Sure Thing, Closer, Transformers, Match Point...
Anything handled by Technicolor post-NSS (late '90s on) is very hard to get a hold of, while for Disney it is nearly impossible.