I have played a couple games that would give you XP rewards for eluding enemies usually capped at half the XP you would earn from killing them that was subtracted from the XP you would receive from killing them. Lionheart did this and it worked really well. The XP rewarded would vary according to the distance the player was from the enemies, and as that happened the chances of them spotting the player also increased -if I am remembering correctly, it has been years since I played that title. Most games don't bother with that sort of thing, the reward of stealth is usually being able to see what sort of fight you are in for and being able to prepare accordingly which is fine as that can lead to drastically reducing the difficulty of many encounters in and of itself
