One of my favorite bits of hidden music trivia is the case of Electra Glide in Blue.
https://en.wikipedia.org/wiki/Electra_Glide_in_Blue
This 1973 movie was directed by James William Guercio, who was also the producer of the band Chicago's first eleven studio albums. He also founded Caribou Ranch, the famous recording studio in Colorado at which many a hit album was recorded. The studio was closed after it was damaged in a fire in March 1985.
https://en.wikipedia.org/wiki/Caribou_Ranch
The film starred the actor Robert Blake, once of Baretta fame and more infamously accused in the 2001 murder of his second wife.
The soundtrack of the film features several uncredited performances by the band Chicago, and any fan of that band might wish to seek out the soundtrack, which I have loved since seeing the movie in the theater in 1973.
Here is one clip from the film with some of the music by Chicago.
Also, several band members appear in the film in bit parts including:
Peter Cetera as Bob Zemko
Terry Kath as Killer
Lee Loughnane as Pig Man
Walter Parazaider as Loose Lips
Finally, this is the third time I have posted about the film on QQ, including this post in 2015:
https://www.quadraphonicquad.com/forums/threads/movie-music-and-music-in-movies.20681/post-250725
I have been enjoying this thread, thanks for all the fun contributions!