Ah, I finally see it. In the image below I took the effort to identify and display the assorted one way markings.
Click on it to enlarge it.
The best one-way marks are usually at the corners of the card to allow for easy spotting, but sometimes spotting them is TOO easy and even laypeople can spot them. These marks aren't the best hidden ones I've seen, but they're still small enough that it's unlikely anyone would spot them accidentally during your routine or poker game.