Sneak is based solely on the sneak skill, correct? No NPC line of sight, no light sources, just straight up skill numbers?

Secondly, when I'm hidden and I open a container, steal something and exit the container why do I get caught? The hidden icon is there the whole time until I exit the container. Is there something else at play here? Does a higher sneak skill prevent this from happening? As a reference, I was in the Seyda Neen Warehouse and a guard caught me stealing from the upstairs containers while he was downstairs and on the other side of the building.

As it happens, I started a theivey character for the first time since I got back into this game and I'm finding it impossible to use her. I have no indication of when I'll be successful or not, my combat skills are pathetic, and I have no magic ability in the slightest. Not a very easy character to play as.