Cheers all. I think I will email mr Nicol first try and see what he thinks, and if he has seen this issue before.
No, it is not an alloy collar, the top part of the tube (above the weld) is only ti, the weld seems to be at the junction of that part which is thicker and the bottom part of the tube, which goes down to the BB, and is thinner.
There is no light poking with a torch, as the crack is really thin. From the inside, the difference between wall thicknesses is obvious - there is a straight-edged gap which you can feel with the finger, just as if it was two different tubes end to end.
24p says you don't weld ti tubes end to end, so there might be an overlap - but I am not sure. The crack might even only be on the weld itself, but not through the whole tube, if overlap there is.