Red was starting to get worried when Cat still hasn't noticed her even when a few minutes. He knew that one of Cat's greatest strengths is her superbly otherworldly detection skills when it comes to humans. If there was someone sneaking up on her, she would notice it in a few seconds. And if someone were to direct bloodlust towards her, she would notice it even faster.
That being the case, Red was wondering why Cat has not noticed him all this time. He was thinking that maybe Cat had gotten soft and her detecting skills now are lacking.
However, as it turned out, Cat noticed his presence all along. She just didn't mention it to the other lookouts. As for why she did so, she was probably testing the lookouts and decided that Red would be most fitting in the experiment.
"So that's why she didn't say anything and just let me be, huh." Red smiled to himself. To his relief, Cat still hasn't lost her touch when it comes to detecting people.
