@pointlessone@status.pointless.one Wetting the stained surfaces with IPA and letting it evaporate did not work, but wetting them with IPA and then rubbing down with a soft cloth (actually kleenex) worked. This doesn't seem to have affected the texture of the plastic. So I think it must have been some kind of residue after all, but soluble in IPA and *not* in vinegar ... skin oils + plasticizers + detergent + hard water could make a lot of things.