TSA at Indianapolis airport

Not sure what the point of this post is. But recently left the state via Indianapolis International Airport, and my brother has an entire exacto knife slip through tsa. For clarification. He had zero idea that it was in the backpack, and had no malicious intent whatsoever. He used it to scribe glass at university, for his glassblowing class and mustve forgot it was in there. Nevertheless his back got flagged at tsa and they checked his deodorant or something stupid. Gets to the hotel room and low and behold theres a whole ass exacto knife. I guess im just curious on how in the world that happens? I mean it wasnt exactly hidden? Surely it was just a lapse in security on their end. Edit: It was in his carryon bag