When someone says "Letter left at the stand." or a "Letter left at the Blue Mushroom, left on the dusty counter." Doesn't that mean anyone that steps in the Blue mushroom can see it? I mean, it is kinda obvious that an adventurer goes into the blue mushroom and "Finds out" about the cult of the hidden one operating in Sanctuary, Before the Svifneblin picks the letter up.

Letters left at populated (or, in the Blue Mushroom's case, observed) areas would require DM assistance in stealing a look at notes left there - which could be questionable in the first place unless you can prove you observed someone delivering the letter and have a reason to try to read their mail.