Originally Posted by Andrew R Stewart

Remove the BB. Place a long wood shaft in shell. Try to flex shell up and down while looking at "crack" if the tube is cracked you'll likely see some light through the expanding crack (a small flashlight stuck in the seat tube will provide the light source).

Or clean the inside of the seat tube well then apply a thick lube to that inside surface where the crack would be. Ride the bike. Do you see anl lube seeping out through the crack?

Given that the crack seems to shift it's path and go up the shoreline, where there's usually a thin layer of filler, makes me think the filler has failed. This is seen every so often from poor pre brazing prep or a cold brazing job. Andy.