Excellent defense: "You sent me the packets revealing where all the other players were. If you didn't want me to know they were behind walls why did you tell me precisely where they were?"
Yeah, doing such checks on the server side of things is more computationally intensive but it would solve that problem entirely and you wouldn't need client-side anti-cheat bullshit anymore.
The first rule of network programming is never trust the client. How does anti-cheat software work? By trusting the client.