It was a EZ Bass track. So an instrument track, but not MIDI. The instrument was gone.
And no alert.
I've been using computers forever and weird stuff happens. Logic, Final Cut, Premiere, and more since the '90s.
I've been using 10.7.5 and that (restored from saved) project for a few days nonstop. There have been no more weird things like that. I did Disk First Aid on all my discs, and even checked for viruses (yeah, pretty unlikely). I also zapped the PRAM since it's an Intel Mac.
If it goes bad again, I'll do a clean system install and restore files from backups.