Been thinking about mapping more. Once upon a time, my players encountered a rival adventuring party in the dungeon, fought them, and took their stuff. At the time it didn't occur to either them or me that that rival party probably ought to have had a map of parts of the dungeon that they had explored.
On the one hand, having rival adventuring parties be a source of intel in addition to magic items makes them a great target. Stealing their maps might even qualify as a good use for my least favorite thief skill, Picking Pockets, and maps could be a good target for skullduggery in town too rather than just in the dungeon. I also like that this would allow a DM to provide information to the party about the layout of the dungeon, but there's a good reason for that information to be unreliable - just as the party may make mapping mistakes, so too their rivals. This casts the players' map into doubt and requires them to resolve inconsistencies. Some of my favorite mapping moments in previous campaigns have been when multiple players had their own maps and have had to build a consensus map out of their fragments, and having NPC maps as an additional source seems likely to lead to similar moments.
But on the other hand, figuring out what parts of the dungeon each rival party has explored and building unreliable map-fragments sounds like a pain in the ass. It's the sort of thing that I would probably want to do after the session in which the party captured the map (ie, random encounter result of Rival Party, PCs capture the map, session ends, I figure out where exactly they had been and what they knew). Maybe this is a good reason for OD&D's Read Languages to specify that it can be used to read maps - "Oh you didn't prepare Read Languages today? I guess you can't read the map and will have to do it after the session." Maybe it's normal for adventurers to keep their maps encoded, so that when captured you need to bring it back to town and spend some time deciphering it. I dunno.
I wonder if the intent for treasure maps in OD&D was for them to be maps of parts of the dungeon, rather than maps pointing to wilderness lairs like I have used them in ACKS. Looking at the text again it really doesn't say.