I had never considered this topic to be an issue until a couple days ago when I played an interesting game. My opponent had a saboteur on a shuttle and insisted that he had the right to look at my face-down damage cards prior to picking which ship to target with the saboteur, or indeed if he would pick a different action for the shuttle entirely.
I didn't think the rules forbid the action. Also, I doubted it could make much difference, so I let him look at the cards. After all, how often in a game will there be multiple damaged targets within range 1 of the shuttle at the same time?
I think there was only one instance in the game where he looked at the damage card on my ship and then chose a different action instead of the saboteur. The card was a munitions failure on an X-Wing without a secondary weapon installed, so of course the sabotage would have been pointless.
I don't know that the face-down damage cards are really meant to be secret. My assumption has always been that face-up or face-down is simply a means to distinguish between critical and non-critical damage results. Still, I couldn't quite shake the feeling that peeking at the cards to gain an edge during the game was kind of a d-bag tactic.
Have you ever encountered this before? Is it normal and I just haven't seen it before?