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The ace I mentioned near the end of my report, that I ignored, looked like this. It was the least obvious. On the others, the crease was longer and deeper, but yes, this is about how the marks looked.

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Hmm. Well, I had an ongoing situation with these marks showing up in my home games, usually on aces and other random cards. It didn't seem to be of any use to anybody. "Ooh, he might have an ace of spades! Or a trey of clubs! Or a seven of hearts! Or..." It was weighted to big cards, but not exclusive to them. And there's no discernible pattern.

After some Matlocking and swapping out setups and tracking who was at what table when, I narrowed it down to two people. By that time I was completely fed up with it, so I let the whole group know what was going on and what I had been doing to monitor it, assuring them that it didn't seem useful at all. I probably should have waited 'til I narrowed it to one person, but even then you can't prove a negative for everybody else. And I was tired of buying new setups.

I couldn't easily reproduce it, but I was trying with a fingernail. Some have suggested that perhaps people are enthusiastically checking on their cards with a chip in their hand. I find it hard to believe it's accidental, but it could just be someone with sharp fingernails excited about a big hand. In the end we all figured, if it was intentional, it was about the stupidest form of marking ever. Useful marking would be on the edges in various locations, not randomly in the middle.

Since then, I've noticed similar marks on a lot of cards in the cardroom. They're everywhere. And they're of no use.

tl;dr - don't worry about it.