Disclaimer: I don't own SVU.
Author's Note- this is just a little idea I got after watching the first scene in Personal Fouls. Slight spoilers for the episodes Undercover and Pursuit.
Fin was watching her.
He was always watching, always looking out for her. She was like the younger sister he never had, though she would hate being called a "little sister". She acted so tough all the time, but he knew better.
He could see how the captain's command hurt her; he could see the heartbreak on her face as she packed up her old partner's desk. Fin knew that Elliot was gone, and he wasn't coming back. But Olivia still hoped, and wondered why he left. She didn't want to believe that he left SVU; that he left her. Fin was extremely aware of how deep her feelings went for her ex-partner, and how much he liked her too. His attraction to her was most definitely the reason he left; he couldn't attempt to remain faithful to his wife when he really loved Olivia.
Fin watched Olivia pick up a picture of Elliot and hold it for a while and he could practically see her throat tightening, trying to reign in the tears that wanted to fall, but wouldn't let them. Again, hiding behind her tough persona, where nothing and nobody could hurt her, and she didn't need someone looking out for her. But she did.
That time she was undercover at Sealview and was almost raped until he came in to save her was one of the scariest moments in his life. He remembered talking to the other inmate, and how panicked he felt when she told him that Olivia went down to the basement, alone. After that, he promised himself that he would never let anything like that happen to her again. When he told Elliot that he would "have her back" he completely meant it- he just couldn't resist adding that comment that he watched her back and not her backside, like Elliot did. He was joking though, Elliot was a great partner for her, and Fin trusted him that he would be there for Olivia.
But right now, here she was, without Elliot, her partner of twelve years who was openly acknowledged to protect her, and she him.
So Fin watched her, silently protecting her from the sidelines.