Greetings,
As far as I can recall, you are the first to mention this.  I am not aware of any card manufacturer that use grease on their cards.  Thermal paste yes.  Paste has a high heat threshold, and it would have to be overapplied or get extremely *hot* for a sustained period to come out.  Several of my cards have stickers with serial numbers on their backplates, but none have ever come off. Some are over 5 yrs old.