When a girl says "night night" to a guy, it typically indicates that she is saying goodbye or goodnight in a friendly or affectionate manner. It's a casual and sweet way of expressing good wishes for the evening or bedtime.
In some cases, using terms like "night night" might suggest a level of comfort and closeness in the relationship, particularly if the individuals are friends or have a warm rapport. However, it doesn't necessarily imply romantic interest on its own.
Context and tone are essential in interpreting the meaning behind such expressions. If the girl consistently uses affectionate language and displays other signs of romantic interest, it might be worth considering her feelings further. Otherwise, taking "night night" at face value is likely sufficient, and it's merely a friendly way of wishing someone a good night.