Are you asking, "what specifically in this one page of manga is making some people say red flag?"
Or are you asking, "what is a red flag?"
Cause you can just Google that.
As for the former, I don't know. There might not be. Death flag, yeah I can see that, but red flag? Not so much. Unless jirai-kei is just an automatic red flag to some people.
But as I like to pretend to say, "never attribute to foreshadowing what may be explained by the author's fetishes." Lol