Ranged divination effects (monster detection, object detection, clairvoyance, maybe telepathy) are suppressed if the level is not mappable.

The spell of clairvoyance grants a few hundred turns of intrinsic clairvoyance when cast at Skilled. This makes it useful for Samurai in particular.

Premonition system: under various conditions (crystal balls, clairvoyance, dreaming while you sleep, the Oracle) you can receive a premonition of something that will appear later in your game (this could be almost anything: a monster, object, dungeon feature, special room, etc) which then has a very high chance of generating subsequently.

If you are a Caveman, you get telepathy or clairvoyance while you are hallucinating.