Hi!
A fellow judge has detected a way to mark the cards taking advantage of the sleeves that are a bit transparent. Thanks that he has shared it with us, we can be aware of a mark that is quite imperceptible if it is not deliberately sought.
It is important to keep in mind that these types of marks are legal as long as the card can not be identified with sleeves. There is no problem if someone bought a card with a mark but it can not be spotted.
Few examples:
This kind of marking is very subtle:
When it is in a bit transparent sleeves, we can identify the white mana symbol in the unmarked card, which is not seen in the marked card.
So, just like when you saw this type of sleeves you check carefully for double face cards, keep an eye on this kind of markings from time to time to avoid bad temptations.
Best regards ^^