A search can be really helpful:
That was very, very helpful!
I struggled with cdrdao of which I thought it would do the trick, but it’s that easy: cdparanoia -B and the pregap track is caught in a separate file.
Thanks to @chirlu for this hint!
I created a pregap track CD uploaded the TOC to test.musicbrainz.org. If there’s a hidden content, it shows up in the TOC details. The track 1 offset has to be more than 2 seconds. (https://test.musicbrainz.org/cdtoc/3MUS2lL2oSIKNzHZL4Qv6pbCTd4-). So I would not miss it, if there is something. And with cdparanoia it’s no problem to get it into a file.