How hidden is “Hidden” in Dresden Files

Sally the thief sees guardsman Bob. They are in the same zone, Sally having just rounded the corner. By default, both are aware of each other. If Sally rolls a block using stealth against Bob’s Alertness, then the Bob does not see her. On the other hand, Sally could maneuver, trying to place the aspect "Hidden" on Bob. This would let her tag Hidden later, probably for some kind of sneak attack.

I have a Stealth-focused character in my campaign. She has been using the first, then the second, in order. Last session, she asked what happens if she skips that first step. My ruling at the time was that she was staying in shadows or otherwise concealing herself, but that the guard knew that somebody was there. She could invoke that aspect to disappear completely from view, representing his losing track of her.

Thinking about it, I’m not sure about this. Should someone who has the aspect "Hidden" be completely unknown? When I have NPCs hiding from players, I tell the players who’s hiding from them and usually where they are, and trust the players to stay in character. (It usually works pretty well.) That gets run as a block, and they tend to place the aspect "Concealed" when they’re darting around shadows making a hard target of themselves. On the other hand, someone with the aspect "on fire" is still on fire, whether compelled or not – they just don’t have any mechanics associated with this. So someone who was "Hidden" could still be unseen.

How should these different versions of hidden be handled?