Never seen that one. I have heard of stress cracks in MC frames but this looks like it took a severe hit. What year is the bike?
I'm thinking the same thing, and also never seen this pop up before..
it does look like it took a severe whack to the lower (front, left in photo) part, as that area has some darker residue, like aging, to it.. it probably got whacked hard on a curb or something, and after time the crack grew and finally failed (as seen by the clean metal at the rear and top).
tough to predict when it occurred, but I'd bet it's the reason it got sold...
bummer, best of luck getting this taken care of...