I remember that when I was young that a storekeeper and a veterinary were arrested because they were reading the unsold scratchable lottery tickets of the former with the X-ray machine of the latter. They kept winner tickets for themselves, and the the rest to the unsuspecting clients of the shop.

Are you sure that the PrintCoin seal is also opaque in non-visible wave lengths? Or that the black ink isn't more opaque than the seal? It's all about contrast.