Hovering over unexplored squares reveals if they are walls
The mouse cursor changes when hovering over unexplored walls vs open areas. This lets you know the basic layout of the whole dungeon without exploring it (particularly useful in the VDDL).
It does more than that, it will also reveal where monsters are on the map. Earlier today I noticed this behavior (but forgot to screenshot) when it wouldn't let me move to stairs. I noticed that my mouse cursor had the sword icon instead of the boot. Teleporting into the room confirmed a monster was on the stairs. I was able to recreate and have uploaded the screenshots. You can clearly see a sword in the one shot and a boot in the other. Moving up shows the monster (though it did move).