I was at the game and the receiver made a very quick and brief raise of his hand, I'm not sure he ever fully extended his arm. The ref near him did not even see him raise his arm and is not the one that threw the flag.

According to the rule , the returner must make a clear and obvious signal by waving one arm clearly above their head, fully extended, and waving it from side to side.

The kick returner definitely did not fully extend it and waive it from side to side.

Kind of a tough call to make when receivers somewhat lift their arm but don't really extend fully or wave it. And some kick returners have taken advantage of that in the past.