Today I noticed someone taking something discretely when it was darkness (and I was blind). Doesn't seems logic. Should be changed so that only those who can actually see the person who loots, could notice him.

You do roll a spot check, and it does do a line-of-sight check. As of v820, it will now -also- not alert you if you're blind or asleep; I would expect people to be able to just behave intelligently in those cases, though.