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I told Serana to wait at whiterun at the front gate, I forgot about her and when I came back she wasn't there, where would she be?

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Go back to where you first obtained Serana as a follower, she will probably be there.
Followers do this thing where if you leave them waiting for long enough they will go "meh, I'm tired of this" and go back to where you obtained them, or in the case of a follower you obtained through thaneship then they will be in your house if you have a house in the city you're a thane in.


tl;dr:
Check where you found her or your house.

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