I've had instances of players looking at their decks and moving cards around without the other person knowing.
I tried some tests with friends, and this is actually possible!
Disconnect your internet -> Look at whiole deck before OCTGN can dc-lock the screen ->Reconnect
The message that you looked at deck never shows up!
So you got the deck window open and can freely move cards around in it, change the top card, arrange them etc.
Can this be rectified? The easiest way would be to generate messages when cards in the deck are dragged around, I think. Or to somehow check and show if a player is looking at their deck.
Good find, we'll look into this.