Originally Posted by starbase63
One would presume the obvious, that she was in the brig.
I doubt Chekov would have trusted her to be confined to quarters.
There might have been "other" unknown conspirators on the Enterprise that could have helped her to escape. I'm guessing that she was in personal custody of a trusted command officer that did not immediately beam down with the first landing team. Chekov did beam down with Kirk's group. I see Spock, Uhura, Scotty, and Bones appearing in the landing party at beam down. Valeris did not appear until Cartwright is told to "arrest yourself!" She then appears with Spock. My guess is that she was beamed down along with Sulu's landing party. To have shown up any sooner in the chaos that was happening, she could have made her own escape. No one really knew who was actively involved with the conspiracy. There were probably others at the conference eager to get away also.
Just my best guess.