I found this 'scratch' on my carbon frame on top of the front derailleur. Not quite sure when or how it appeared. Could this be more than a scratch?
That's just a light scratch. No issues.
if you are worried that the carbon-fiber composite has been damaged by an impact or scratch, the abrasion is far too shallow for that. It has not even penetrated through the paint.
It's not a crack because cracks do no start in the middle of frame tubes, they propagate from edges or holes.
It's very unlikely to be a crack because it is the same width throughout (cracks tend to be wider in the centre, or at least irregular width) and because it is straight, not jagged.
The shallow depth is also a clue that it is a scratch. You can shine a focussed flashlight into it and see if it reflects from the bottom. If it does, then a scratch is the likely explanation. A crack's ratio of depth to width is much larger, so it looks darker in the centre.