There are rules about attending practices in person or using electronic devices to aid in sign stealing. But if you just notice your opponent using a specific sign and it correlating certain plays or schemes, that’s just part of the game. It’s a team’s responsibility to obfuscate their signs, otherwise why use signs at all? If you weren’t allowed to see what the sideline is signaling to the players, just hold the play name up on a card, then you could save time teaching kids the signals.

I’m guessing the investigation is more to make sure they didn’t use one of the outlawed methods, and I’m willing to bet they didn’t and just got pretty good at figuring out signs. Or are just good enough that teams think that’s what they’re doing.