Small steel ball inside
When I opened my ipod g4 I found a small steel ball inside.
"Check to see if it is a steel ball or a drop of solder. I do not think a steel ball belongs in the ipod.
The drop of solder would not have been left in the unit. Inspection should have caught any foreign material, steel ball or drop of solder.
I suspect that a short in the charging system, the only place enough current exists in the unit to melt solder, may have taken out a component. The result may have been a current draw sufficient to destroy enough circuit material to leave the residue you speak of. I think such destruction will be obvious and you will find the culprit.
Good luck and as my son says, ""keep smiling!"""