Unidentifiable sub track on mkv video

Usually mkvinfo lists all tracks in a mkv container, but recently I’ve seen one with a soft subtitle track, selectable in smplayer, but that doesn’t appear in mkvtoolnix gui nor in vlc.

mkvinfo (and also mkvmerge) only list video and audio tracks. Trying to force extraction with

mkvextract tracks movie.mkv 2:subs

yields an empty file.

Handbrake subtitle list, instead of showing a sub track, says it’s a “Foreign Audio Search”, whatever it means.

The sub plays in smplayer with personalized positioning, i.e., to the left if talking person is left on the video, etc, so AFAIK it’s not a srt.

Any ideas on what kind of sub stream this is, and how to extract it?