There was a time, in the not-so-distant past, that the common man didn't read or write. Like in the 1800's, it was not uncommon for a guy to sign his name with nothing but an "X". Or some other such scribble or letter he'd penned or learned.
So for those people who think random letters on the wall or tree some where mean "treasure", are going way overboard with treasure superstitions. And since when are treasures necessarily marked with a mark of any sort at all ? I mean, if you're intending to HIDE something, it would seem rather silly to then go put a mark there for the world to know, eh ?