The customary answer to the question would be prefaced with a "Yes." The responses would then be
a) Yes, I've seen her in the hallway
b) Yes, I saw her in the hallway
Because the event of "seeing" is given a place frame ("in the hallway"), it is more likely that the speaker would use the simple past, to denote an event that took place at a specific location, and by implication at a specific time, making the event totally past.
A rather unidiomatic answer could use the present perfect, by diving the information into two statements:
Yes, I've seen her. She was in the hallway.
This answer is unidiomatic because it is too formal for the situation.
I should point out that this particular question, "Have you seen Jane," is understood to have been asked in order to establish whether or not "Jane" is present. This same question, "Have you seen (X)?" does not always mean this. It can mean something different, as in
I. (at an antiques auction)
A: Have you seen all the antique clocks that are for auction?
B: Yes, and some of them are very tempting.
II. (At an annual office party)
A: Ho-hum, another annual office party. Have you seen the notorious Mr. X?
B: Oh, yes, and he's making a fool of himself with the women again!
These questions in the present perfect do not ask about whether someone or something is present, they ask whether the other person has had the experience of seeing the person or thing in question.
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