My experience with carbon cracking is that it cracks creating an irregular edge, roughly around the warp and the fill (fiber weave) of the cloth. What your situation looks like is a yarn of fiber got moved out of place or accidentally stuck in the layup before the vacuum was drawn and the layup cured. It is kind of hard to tell from the photo. It could crack cleanly like that if there was a hard metal edge, like a reinforcing sleave, under the layup or if this is a seat stay and the seat post ends right at the point where the crack is. To get a crack like that on a composite layup usually takes a very hard edged load.
One way to be sure is to put a little spot of white hard paint (enamel) across the crack.If, after you ride, the crack cannot be seen through the paint, it is a blemish in the carbon layup.Be sure to use a hard enamel and let it dry well before riding.
I had a case where an 18" long crack in a carbon fiber mast did not show until several months later because the paint on the mast was more flexible than the carbon layup. Not until dirt got in the crack could I actually see it, and even then not until I lowered the mast. The crack was zig-zagged. Of all of the more than a dozen carbon failures I have seen, they all left a jagged edge. This included a failure involving an aluminum backing plate/block with a very straight edged load.
