A hairline crack is more likely to be a witness crack, a tiny crack in the paint that appears because the paint is so thick and brittle and carbon frames flex a lot.
Here’s a pic of the witness crack in my 2007 Roubaix, which has extremely thick, hard and brittle paint. This one hasn’t grown in 3500 miles of hard climbing, cattle grids and rough top-dressed roads. There’s another one down the fork crown, which looks really frightening but also hasn’t progressed in 3500 miles. Both cracks are non-continuous, composed of several almost parallel cracks and in both places pulling and pushing the frame hard in all directions doesn’t produce any sound at all so I’m still riding my carbon bike with complete confidence.