I must say it does look as though the tube itself is cracked, but I certainly agree with REKIBorter that cutting the paint off is the thing to do to be sure.
I'd have to agree, seen a fair few cracked frames in my time and they do seem to crack just behind a weld like that more often than in the middle of a weld.
Hopefully it's just a crack in the paint as it's a nice bike and looks like a lot of work went into it.
I'm not sure that a 8-5-8 (i.e., 8 to 5!) tube is all that flexy, but I find it a bit surprising that a crack like that would occur such a long time after the welding that would have caused it. That bit of tube was subjected to an extremely high temperature, which can cause excessive stress, but then again it is designed to take it (and if 8-5-8 isn't up to it, then it doesn't say much for my Kilauea which has a 7-4-7 top tube!)
I think it's a bit suspicious that it has happened just after the frame has been painted. Paint baking is nothing like as hot as welding, and again shouldn't really cause any problem, but even so it gets put in an oven for the first time in 15 years and next thing you know it's got a crack. There's no such thing as a coincidence, I always say. But then again, I'm often wrong.
Was the frame not spray painted? Just going by the primer pic and the mentions of how easily the paint chips.