The problem is this: if you set the DC too high, then he can't find it if he does a full-cavity search on me.
The closest DC in the rules to finding notes hidden in a sword-scabbard is probably 15, for "ransack a chest" -- if you actually look inside, the notes are obvious.
Your interpretation of the rules doesn't, as near as I can tell, allow for separate DCs for searching for things by rooting around through them, and searching for things by looking at them. So either the guard easily finds the plans by looking at me, or the guard can't find the plans if he cuts my sword-scabbard apart and looks inside.
Or am I missing where you assign different DCs based on different search techniques?
Daniel