Why does my cat stare at the backyard fence?

For the last year and a half, whenever I would let my cat out she would just sit in front of a fence and stare at it in our backyard. She does this all the time and never leaves! I don't understand why she's been doing it. There are a couple bushes and shrubs around the fence and I'm thinking maybe there's a plant growing that' scent attracts her to it, but as far as I know I don't see her sniffing around it. She just sits and stares! does anyone know?