Just been round to see a neighbour who's a very experienced roadie and bike builder and he says it's just at the point where the ali dropout enters the carbon tube so you would expect to find some flexing there. It is probably the most stressed point on any bike frame, apart from the forks. There seems to be an accumulation of long-term dirt in the crack, which would tell me it was quite long-established. If it was new with fresh paint or carbon visible I'd be blaming it on the new stiffer wheels I've just bought.
I've tried squeezing the dropouts together and there's no sign of movement or creaking sound.
Hmm, bearing in mind the history of the bike, I wonder if there was some hidden damage when it was chucked off that bridge? Hopefully not because it's a nice bike, but I'd definitely want it checked out properly. You wouldn't want the thing to snap off on a fast descent!
The crack suggests the structure of the dropout to look like this.
The crack indicates that there has been relative movement between the dropout and the horizontal stay. The question, then, is whether it is reasonable to expect this particular joint to have been as solid as a weld.
From the way it is constructed and finished, I would expect the answer is yes. As such I agree with what Ianrauk suggested.
I think ColinJ is right; I think the bike was thrown off the bridge and landed on its back wheel, which was badly bent. If the back wheel took the impact, that could have shocked the lug where it enters the chainstay.
Another update: just spoken with this bloke: http://www.carboncyclerepairs.co.uk/ who says it's almost certainly what he called a "witness crack" in the very thick brittle paint and lacquer layer. Says he's seen loads like this and had loads of panicky phone calls from blokes like me. He says that if the manufacturer didn't paint the joint between the lug and the frame nobody would have these scares. He says that inevitably there's flexion at the joint and that unless it spreads or I hear creaking it's nothing to worry about.
I will wait and see what Specialized say about it on Monday and report back.
Thought forum members might be interested in an update on this, since it could affect a few of us with carbon frames.
After three months of heavy summer use including banging across many cattle grids, some rough top-dressed tarmac and some big out-of-the-saddle climbs the crack has not grown or changed one little bit; it is absolutely identical in size and shape to the picture. There is no movement or creaking so I feel increasingly confident that the carbon specialist's diagnosis of a "witness crack" was correct.