From my experiences this was often the case. I opened many chests right in front of their owners quite often, the only time I was busted on the spot was when I stole from the orc merchant in Cape Dun. Aside from that, telepathy, not visual confirmation was the means by which I was caught.
You may notice that on occasion a guard will stop you after a rampant stealing spree. Even though the accosting guard did not see you, or even if nobody saw you, you will be stopped. He'll say something to the effect of "Some things have gone missing, you wouldn't happen to know something about that would you?" and depending on your thieving skill you will either deny it outright or be given a list of responses. Depending on the response you choose the guard will either attack you or leave you as a suspect. From what I have seen frequent stealing leads to frequent questioning; the more you are questioned in a town the more difficult it becomes to have a positive outcome when questioned.
As you may have guessed this sequence is governed by your thieving skill level as well. High levels will have the guard swallowing your bogus excuses whereas low levels will have him swinging his sword at you.
The only way to avoid such an attack would be to have a relatively high thieving skill. No matter how well you remain hidden when stealing you will eventually be questioned by a guard. Likewise, (well at least in my experiences) you will only be questioned if you have stolen something; that is, you will never be falsely accused. In addition, one chest will go unnoticed, but 10 will have the guards growling at you.
It's a big difference from G1 and G2, one which I personally didn't like.