Is this just the paint cracks? It is on the dropout of my new tarmac, one month old.

The shop owner said "It's normal for this side to crack it might be from the chain"

But when I did test the chain it didn't touch the frame (there got 5mm gap between). But I don't know exactly that when I ride.

So has this happen to anyone?

And how do I know if my frame has cracked or not?
It looks like a chain jam from a wrongly adjusted rear mechanism. The paint finishes on these bikes tends to be fairly hard and a good smack will chip the paint.

The next question is what's been done to the frame. Without testing, x ray type inspection, history etc. Its impossible to tell. So that just leaves whether you trust it enough not to lay out for a new frame. Me, I'd trust that sort of catch if there is nothing obvious in the carbon layup and it rides ok. Go easy to start, build confidence in it; or not. You decide.