Halva smells like play-dough? a common ingredient?
Are you sure it was a sesame based Halva? Halva generally falls in to two wide varieties, one flour based and the other nut based. I would assume that the smell of play-dough comes from the flour. So it's possible you had a flour based one. 

BTW, it's wikipedia that's defining Tahini as being nut based, I'm not so sure about that classification myself.