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No, not every round, only when circumstances change.
1. Dj'hân moves from the rucksack to the stairs. Most people are otherwise occupied.
2. Devargo approaches the stairs where Dj'hân is standing. Dj'hân manages to slip out of sight in time. From here, he can follow Devargo to his room without problem, since Devargo hasn't noticed him and isn't looking for him.
3. Dj'hân finds a new hiding place under the table. Devargo turns around and walks past Dj'hân's hiding place.
Now, I wouldn't necessarily make checks all the time, but Devargo is depicted as a very shrewd and canny individual. I think he'd pick up if something was out of place in one of his own rooms, even if subconsciously.
As for unlucky rolls, well, yes, but that's the same for anything in a d20 system. The chance of failure makes the risks more worthwhile. You wouldn't want sneaking into the inner sanctum of a ruthless crime lord to be without risk, would you?
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