A long silence ends and another passage begins, but the printed track list offers no title. The useful first step is to document where it appears on your particular copy.
Record the observable location
Note the preceding listed track, the approximate elapsed time and the format you played. State whether the passage is a separate selectable track or part of a longer file. Avoid importing a title from memory unless you can identify its source.
Keep edition differences possible
An unlisted passage on one version may be absent, separately indexed or openly named on another. Link your note to the catalogue details of your copy rather than making a claim about every issue of the album.
When sharing the discovery, mention that it concerns the version you checked. A friend who hears nothing may have a different release, not a faulty player. A short, reproducible location note is more valuable than a dramatic claim that you found a secret nobody else knows about. It lets the next listening session begin from evidence.