I'm going to guess around the drain opening.
1)P.O. plug was installed to tight.
2)Drain opening was not perfectly round causing the PO plug not to sit square in the hole
3) Bad make of sink
4)Kids dropped something in the sink
5) ect.............
It's a crack 4" long a 1/4" away from the drain. I was worried about the mounting bolts until the home owner told me this (he isn't blaming me thank goodness) . I don't think I tightened the drain much more than hand tight (it's pretty hard to over tighten the drain right?)
Thanks for your help.
You can "overtighten" a drain, but sometimes it is necessary to prevent a leak. Installing it just past handtight, would almost always guarantee a leak. That location, and depending on where it started and the direction, would usually indicate a weak spot in the sink and the drain caused stress that cracked it.