Here's a hypothetical situation:
Musician came across a trumpet concerto that Musician wants to use for a competition to win the chance to perform it with a local orchestra. There is a well-known and published transcription of the concerto for trumpet and piano, which Musician purchased, and an original version for trumpet and full orchestra. Musician reached out to the publisher to ask about purchasing the music for full orchestra, and they informed Musician that it was never published and only exists as a handwritten manuscript in a library, and after emailing multiple Belgian libraries, Musician finally found the one that has the manuscript and is willing to digitize it and send a scan to Musician.
Musician's understanding is that (a) the music is protected under copyright as soon as it is written down and (b) Musician needs to get a separate license/permission from the copyright holder to perform the music. At the same time, though, after telling the publisher about the original manuscript, the publisher told Musician that it looked at SABAM's website (which appears to be the Belgian equivalent of ASCAP) and that the version of the concerto for trumpet and orchestra is not currently registered, and that it was never declared or was lost.
While it was composed in 1938, the composer died in 1970 which may mean that all his work should be protected under copyright (death + 70 years). At the same time, Musician also did some genealogical research on the composer, who appears to have no children himself and was an only child, so Musician cannot reach out to any direct family who might have inherited the copyright. Neither the libraries nor the publisher know who Musician could reach out to as the copyright holder, either.
Has Musician done everything that could reasonably be expected to find the proper copyright holder? Tickets to the concert (if I did get to perform it) would be free, so there's no issues of monetization/royalties either.