Day 3: 7:00 P.M.-8:00 P.M. - S3-E7
Character mistake: At 07:03:05 PM the man pointing the gun at Jack is right-handed, but he aims with his left eye, which is his weak one. He keeps his right eye closed as he walks away and no longer needs to aim, which is a dead giveaway he has never used a gun before.
Day 3: 12:00 P.M.-1:00 P.M. - S3-E24
Character mistake: Michelle tells the director that the target car is light blue. However, Chloe informs Jack that it is light green, which is the color of the car shown in the chase scene.
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.
Cubs Fan ★