The Casino Seal is a seal that glows when it detects the use of chakra nearby.
"What does security look like at the casinos?" Wakahisa asked.
"Varies a lot," Hazou said. "I checked three different casinos. […] The third place was the most upscale, but I didn't see anything but civilians guarding it. It did have a bunch of seals scattered around the ceiling and the tables, disguised as patterns in the wood or whatever."
Hazou did not finish developing the seal until Chapter 217B.