With Update 19:
- Invisibility is immunity to Spot unless you are too close (about a human body length)
- Everything is more vigilant. Unless invisible and still, something can hear and detect you faster. They are always looking for you...some better than others.
- Scorpions don't have tremor-sense, but they might see you if you're close enough, invisible or not.
- Enemies now hear ANYTHING you or your hirelings do if your Move Silently score is too low while in stealth or if just running around. If a Bard starts playing songs, they'll really hear them. Summon a hireling and he utters/roars something? That, too.
- Giants have See Invisibility. in "Claw", you need kill only one guy, the warlock. If you're crafty, you can use stealth to re-hide from pursuing giants. You should be able to escape scorpions, too, but that means you lost an optional and are just salvaging the quest.
The "eyes" that you see above your character means that something is honing in on you. Take cover. The more eyes, the more likely you are about to blow your cover. On enemies, their Spot bonuses increase the longer they sense something.
The devs were very serious in giving meaning to the Hide/Move Silently numbers and that sounds and movement do count, however small or whatever makes it. Barrel-smashers beware. You can't just hide in plain sight as long as Rangers with the feat of the same name, which just reduces the Spot bonus and lets the Ranger move in closer.