Day 7: 9:00 AM - 10:00 AM - S7-E2
Visible crew/equipment: While FBI agents are searching for the sniper Jack tells Renee there must be a bad agent, and just as he warns her about who she lets in, the green T-mark is visible on the ground. (00:05:35)
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.