What does it mean when there is a little + 'plus mark' over a note in baroque music?

You are quite right, the "+" sign indicates some kind of ornament, but it is more likely coming from the composer rather than the editors. In editing old music, editors are more likely to write out their intentions.

So, what ornament is intended? Composers in that era left much discretion to the performers. The composer might not have been aware of the specific instrument and its capabilities and limitations. A recorder could be more florid than a cornetto. Likewise, the capabilities and limitations of the specific performer could not be predicted. So, it was just easier to leave things generalized instead of particularized.

Still, it was not a license to run wild. It was always expected that ornamentation would be tasteful and appropriate in the context of the performance.

Most likely, a trill or turn would not have been a good interpretation. Better would have been some kind of mordant... a flip to the next note and back. Which direction? Good question. Which seemed to fit better? There was no hard fast rule.

Incidentally, a common performance practice was to omit ornaments (or at least reduce or simplify them) on the first iteration of a repeated section, and then to apply them liberally the second time through.

Most likely this is to do with the fingering. A + over a note in early editions of Baroque and Classical scores means you're dealing with old English fingering where a + refers to the thumb.

+ thumb

1 Index

2 Middle

3 Ring

4 Pinky

Let's cut to the chase before we settle on a phantom 'diagnosis': which composer, which work, which bar, which stave, which beat, which note, and of course, which edition?

You provide that in a supplementary and then we'll all have something to talk about. Until then, it's all wild, quasi erudite speculation so far. Of that no performance practice is made.

Post the detail and we'll do this again -- better...

All the best,

It usually refers to a quick trill