I use sneak while harvesting my fields. It only levels 1/5th as fast as when sneaking near an enemy, but why not? You can also sneak in crypts when the enemy can't reach you.

As for not being spotted, well, you can't sneak when the enemy is looking at you. You need to do it from behind.

If you are taming boars in your camp you can sneak near them while they are taming, but not after they are tamed. You can level sneak up faster that way.

Sneak to kill Deers to build up Sneak skill the armour boosts the base iirc.
You wont get a rogue level sneak in this game, it means you can try get a 1st hit crit.

To lvl up faster sneak skill you have to sneak some animal or monster with rested buff