We know that buildings can be designed with secret rooms in mind. What this really needs is goods that local polities have good reason to ban; as you say, hidden deadly weapons and the like. If local laws said that every weapon had to be peace bound, and a blacksmith could design blades that, from all appearances looked peace bound, even after use, that could be worth it for an aspiring assassin. No need to worry while in the inn, everyone hear has a peace bound sword. Oh, that man just drew his and rammed it though my heart. Oh well.
I like the idea.