"And I assure you there is a mark on this door—the usual one in the trade, or used to be. Burglar wants a good job, plenty of Excitement, and reasonable Reward, that's how it is usually read. You can say Expert Treasure-hunter instead of Burglar if you like. Some of them do. It's all the same to us. Gandalf told us that there was a man of the sort in these parts looking for a Job at once, and that he had arranged for a meeting here this Wednesday tea-time."
This is a well-known passage by now (and if it isn't it should be). Though hobbits and most folk of Middle Earth seem to be extremely literate, they still make use of a generally pre-literate (or simply secretive) desire to record things in symbols and signs that are recognizable without being able to decode them as alphanumeric. Never again are mentioned the sigils and signs to which Balin is here referring, unless its some dwarven rune and thus incorporated into their actual language.
However, this idea of a secret adventurers code of glyphs ACTUALLY EXISTS.
This seems like a great thing to build on and expand into an adventurer code. Perhaps something that it takes a proficiency to be able to decode. I'll get to it once my group stops adding games for me to design... the newest fling is Dark Sun. We'll see how long that lasts.