WHAT DO THE THIEVES SKILLS ACTUALLY MEAN?
You're playing a Thief. You want to pick a lock. You take a peek at your sheet and see that your chance is 15%. That sucks! Surely you can do better than that?
I think you can. I think the Thief's skills are not meant to replace existing stealth rules (such as they are), but to augment them. This is what I mean:
You try to sneak through a room where some orcs are playing bones in the corner. The dice are thrown and the result is 55%. That's well above the 20% you needed to move silently. What does that mean? It means you didn't move silently. It does NOT mean the orcs heard you. It simply means you made some kind of noise that might be heard. So the DM should then make the normal roll to determine if the orcs heard a noise or not.
If you were a Fighter sneaking through the room you'd only get the second roll, not the first one too. So the Thief has an added layer of rules to cover stealth.
Same for hiding in shadows. A failed skill roll means you could be seen. It does not mean you actually were seen. When the Thieves' skill rolls fail you simply fall back to normal rules such as a surprise check or hear noises.
I don't know if this was how the rules were meant to be used or not but this is how I've been thinking of them for a long time. It makes a thousand times more sense to me now than before and it means I don't necessarily have to house rule the Thief (though in all honesty I still do...).