A wizard has seen through her familiar's eyes an NPC who was within 30 feet of the familiar, but more than 30 feet away from the wizard. Can the spell locate creature, cast by the aforementioned wizard, be used to locate this NPC?
Per the spell description for Locate Creature:
so long as you have seen such a creature up close--within 30 feet--at least once.
The description for Find Familiar says:
Additionally, as an action, you can see through your familiar's eyes and hear what it hears
So as long as you're within 100 ft of your familiar, you can use its senses. So now it's a question of the quality of the familiar's sight. Can it see the target? It it obscured? In darkness but your familiar lacks darkvision? Blurred? Or any other factor?
So as long as it can see the creature and is within 30 ft of the familiar, then you have seen the creature while you were within 30 ft and therefore the spell should have no issue.