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.