Stealth effects should be dependent on the burden level of the being (which they right now are not).
Stealth should work much better close to a wall than compared to the middle of a room.
Stealth should have an active and a passive mode. The active mode provides a higher chance of success but reduces speed by one half or so. Active modes ends when combat starts.
I think also many things upon noticing something like that would get scared and run away (animals especially), or maybe try and alert other monsters nearby.