It's true in reality, but UrW's rules are somewhat different, as you're brought out of stealth when somebody notices you in UrW (and refused from entering stealth when seen by somebody/thing you're unaware of, such as a bird behind your back just after zooming in). However, there are two exceptions to that in UrW that I know of:
- Robber homing. They always find you, regardless of stealth and darkness as far as I've seen, unless you manage to get away by zooming out (which may require a running sprint to get sufficiently far away). I have to admit I haven't done much testing, as my approach is to get out of there as quickly as possible when I see robbers (unless in the elimination stage of a robber quest, where I want to close the distance in a controlled manner with no hirelings in front of my character and not too many of them straggling far behind).
- Njerp aggro homing: When you fire an arrow that hits a previously unaware Njerp he'll run towards you, and change course doing so, even if you're still in stealth and moves away at an angle. Past experience indicates they can lose you in darkness, but probably only after dropping out of aggro.
However, I don't think I've ever been brought out of hiding by being spotted by an unseen critter.