Hiyer, Bernhard!
I think that you mean the mark that would appear to be a depression in the side of the tuber. If that is the case, I think that is where and offset was released, and some rot occurred at the union and then healed.
I have had plants of this species and some Amorphophallus that would occasionally not completely consume the old tuber. However, the remains of the old tuber were always directly below the new tuber. The central primary growing point of the plant is that central point, surrounded by the primary roots,=A0 where the leaf and inflorescence grow up and the tuber shrinks and and regrows from the bottom. This is the apical meristem, and the leaf and inflorescence will always grow from that same spot, year after year.
Remember that as the inflorescence and leaf grow, the tuber shrinks towards the meristem, like a deflating balloon.=A0 It regrows in the same way, expanding outward like a balloon from that same point. It is not like Anchomaenes or Gonatopus, where leaves can grow anywhere over the upper surface of the tuber and the tuber is not completely consumed.