Or could it over-spray from the last paint job, caused by poor use of masking tape?

In case it's not obvious, it's the seat stay joint, between alu and carbon. It's a cheap-ish Asian frame, so I won't be too devastated if you wanna give be bad news, even though I love riding it.

I haven't noticed it before, but I can't remember ever really looking, so I'd say it's been like this for a long time

Looks like it is paint on the epoxy. a small line following bond lines happens occasionally. Seems to happen more on white frames, no real evidence of that though. I wouldn't worry about it.
If any hairline gets wide, with noticeable gap variance with load, or any white corrosion appears, then it's time to worry.

It appears to be a crack in the joint glue to me , aluminuim expands/contracts a lot more than carbon but painting the alloy white should minimise that thermal cycling, I agree with thisisatest that you should keep an eye on it, but the load there should be compressing the joint further together (ie generally safer)

Those carbon/alloy bonds always crack, but they last a ver, very long time after the crack first presents, they tend to just make a lot of noise.

Thanks for the replies. I'll keep my eye and ear on it

Yeah my dad has an old Ribble Dribble Wreck which is the same frame type, it showed a crack around the jointing bond within the first few months, that was 12 years ago. It's still being used and he, like you, loves the ride of it, and just uses it as a bomb to bike, no love lost if it dies, but loves it whilst it's alive!

Just keep using it and keep a check on it like you say.

^ Thanks for the reply. Yes, I love mine, too. It's one of these, but it was branded as a Tri And Run before I resprayed it. God only knows who makes them. I bought it as a cheap 'SH1tter' bike, but I like it so much that I do about have my races on it. It's not super-stiff or particularly light (the frame is 1365 - 57cm effective top tube), but for some reason it feels fast, and I love the fit.

The resin in the carbon fiber is very caustic to aluminum. Any aluminum surface it comes in contact with has to be extensively coated and shielded so the resin doesn't just eat it alive. This was a constant problem not just in the early days of carbon fiber main tubes and aluminum lugs, but into BMCs and other frames that kept both materials in one bike. A manufacturer simply has to get it right, and small cheap Asian carbon framebuilders don't do a good job with this. I suspect you'll find that that line has a bunch of white powder underneath, which is corroded aluminum, and that the problem may progress.