The narrator should be able to give out these cards to a player without the others seeing them. Knowing that another player recieved a "Threatening Letter" asset or "Spy on player X" sublot card could color some of the other players behavior.

Also, if a card is given secretly, it wouldn't show up in the players' card popup box if the other players click on them, but it would show up to visitors, the Narrator, and the player who owns them.