All my characters are - sooner or later - good in sneaking. Thus, they can also pickpocket (pp) from NPCs. At least they should.

I've read about bonusses and penalties when sneaking up to NPCs of higher / lower levels. Nevertheless, when I get the message "hidden", doesn't this mean, I'm not visible at this moment?

Examples:

In Camp McCarren I get the quest to bring the heads of three villains. One of them is Cook Cook and I should talk about this one with a certain guy. During the day this guy is punching a training dummy; he tells me to shoot a certain brahmin etc. At night, when he is asleep, I go into his tent. I crouch, move around a bit until I get the "hidden" message. Now I steal the unique glove (for unarmed fighting) from the SLEEPING guy. My sneak skill is 80, I wear light stuff (brahmin hide), have agility of 9. Nevertheless, I get "caught" immediately, the entire camp goes hostile, the guy wakes up, takes his glove back and starts to attack.

In Novac I can steal the key for the save out of the lady's pockets with a sneak skill of 30 without any problem.

In the Great Khan's camp I can pickpocket the sleeping "Papa Khan" without alerting anyone around. I can even kill him and nobody takes notice.

Question:
Does the game itself prevent me to steal from certain NPCs? In my eyes it looks like "you may steal from this one, but not from that one. If you try the latter, we nerf your skills down to zero and you get caught".

Additional question:
When I try to sneak and pp, will my companions give me away although they do nothing?

Pickpocketing incurs additional penalties depending on item weight and some variability. There may be further modifiers for important items.

just use a silenced weapon stay hidden save before u shoot, aim for the head vats or not use a stealthboy if u want one shot head kill loot his body for what you need...

When the HUD says Hidden, it means the NPCs in the room do not notice you just being there, this does not mean they will not notice you when you take action.

When you attempt to steal from someone who has a higher sneak skill or has a setting to be more aware of attempts (it used to be tied to Security in Oblivion, now I am not sure how they factor it) then the NPC will be more likely to catch you at this. Buff up your Sneak before attempting again. Always Save before you attempt so you can reload. When the reload comes up, move around a bit to make sure you are still hidden and in a good spot. I have noticed that on NPCs that you are supposed to steal from the game does make it easier to pick pocket them, like Cachino. I can be in front of this guy with a low sneak skill, in daylight, in a crowded room, and not set off any alarms.

Yes, your companions will screw up your otherwise stealthy activities ether by being seen or reacting to something or by attacking when they are not supposed to. Now, when I want to be sneaky, I tell my companions to wait, or I send them packing back the Lucky 38.

@ Cat Herder

So, if I understood correctly, you agree to my suspicion, that the game "artificially" raises or lowers your skills, when "it wants" you to pp someone and when "it does not want". Right?

@ Mack

I'm not great in killing people just because I want something of their belongings

It doesn't raise or lower your skills, but the skills of the person you're trying to steal from will factor into your success. Like if you have an 80 sneak and the person you're stealing from has 100 sneak, it will be harder to steal from them. I don't know if that's accurate, but that seems to be what CCNA was saying. Makes sense to me.

Also, I'm quite sure that larger items are harder to get away with stealing. For instance, you're more likely to get away with pickpocketing a key or a couple of caps than you will a weapon or a huge stash of caps.

item weight AND value factor into it.. also its very hard tho not impossible to steal weapons off a person. The weapon is by far the hardest object to steal off a person.

The problem you are having is isolated to Cook-Cook, correct? I was reading (I think on the wiki earlier or maybe the guide) about this quest, and the three quest targets, including Cook-Cook, all have artificially high perception, so you cannot sneak up on them. This is why pick pocketing would fail, even when they are asleep.

When I pickpocket a character I always save after I get the "hidden" indication. That way I can reload as many times as its takes to get it done without getting caught. At the most, with my very low level stealth, it usually only takes me 6 or 7 times to do it successfully. Not sure if you find the save and reload method "not playing the game properly," but it seems to work for me just fine.