Citoyen's broke recently at the chainstay bridge, I think.
The rear triangle has no paint on the inside and it inevitably rusts out in time, although in your case it just looks like the metal got overheated by the brazing operative. Not hugely expensive as a spare part, and you can get the bushes changed while you're at it (tip: get the old ones out yourself and just pay to have the new ones fitted and reamed; this turns a £50 shop job into a £10 job if you're lucky).
You *may* have some luck with a warranty claim if you're the original owner.