It's funny this question pops up now, when we spent hours with Zeg on Kestra to find this song too, yesterday
Anyway, all the musics in the Stoeln Data #6
are in this mysterious Opus format, according to Kris/Anarchy.
I have extracted said musics but didn't try to convert them as 5 of 6 can be found in original format, here. However, as you mention, the menu music is nowhere to be found. I had hoped it would pop up somewhere, but ...
So I guess I'll have to look into this one.
Will keep you updated.