youngprof said:That mark does look like some adhesive residue. Have you tried gently rubbing at it with your thumb (NOT scratching at it), to see if it rolls up and off at all? I had something like that on an MJ bag I bought once, and it came off when I rubbed at it; it seemed like it was a bit of adhesive from someone's hands perhaps (maybe the bag maker him or herself).
I tried again. Still not going anywhere. I have some of that stuff you use to remove adhesive, but I am pretty sure it's not great for leather. It has citrus oil in it.