One of the great things about Onitama is you have all of your cards on the table, literally. You can see every move your opponent could make and your opponent has all of the same information as you do. In this game you share information because it is not about being sneaky and hoping your opponent doesn't notice, it's about being clever and using your cards the best. Sometimes we make a mistake and overlook something and lose a pawn. This is unfortunate but fighting in a dojo does require vigilance. However, winning a game because your opponent didn't realize his master was being attacked feels like a hollow victory.
Why not have an officially declared check, like in chess?
This is a rule in chess for a very good reason, and the same applies to Onitama.
When another player's master is in danger, The Honorable thing to do is remind them of the threat. Whether it is a direct attack or a discovered check, all information should be shared with all players.
I have adopted this policy for IRL games.
This is my unofficial house rule because I don't like to win on a technicality, I prefer to win by pure skill and wits alone.
Do you think this game should update the rules to include declaring check?