And yes, that is a stitch marker knit right into the sock. I make coordinating stitch markers for each of my sock projects, just for fun. When you do toe up, you really don't need them, but I like to use a marker to mark the rounds during the toe increases. After that, they are just "knitting jewelry." When I'm nearly finished with the second sock, my faithful stitch marker, as a reward for stitcking out the whole project, gets knit in right above the ankle. Kinda like a sockish ankle charm. These socks are for me, hence the frog in the marker.
I also knit up a pair of socks for my daughter, who has an affinity for camelids -- alpacas and camels in particular. So here are her socks ---
And here is her special stitch marker. She didn't care if it was 100 degrees -- she pulled them right on and wore them in the house for her whole weekend visit. In fact, she was so grateful, she is probably going to get another pair of socks in the very near future! :)