Vanguard takes screenshots of your PC every time you play a game. Every time you play a game a function is called to screenshot your PC’s screen, in case Vanguard thinks you might have something suspicious, it screenshots your ENTIRE PC screen (all monitors).
Edit: Not trying to spread false info this was shared to me via a friend and there is other data to back up that this is real https://www.unknowncheats.me/forum/anti-cheat-bypass/634974-vanguard-taking-screenshot-pc.html
https://www.unknowncheats.me/forum/valorant/484475-vanguard-screenshots.html
Also there’s a fundamental fog of war of what your anti-cheat can see. Valorant cheaters are using hardware cheats, that literally takes in a video output, analyzes it, and sends in mouse inputs, on a different computer, the anti-cheat straight up can’t see it, they only see there’s a video out and a mouse in. Ultimately, having physical access of a hardware and you can just tell the software whatever you want it to see.