In a newborn I would say she was rooting for food but babies find sucking comforting so it's probable a comfort thing. Plus babies learn a lot more from putting something in their mouth than they do from looking at it. Its a normal developmental stage.
At 13 weeks I would say she has discovered the fact that she can make her hand go in her mouth and is enjoying exploring things with her mouth and seeing what they taste and feel like. At a similar stage I thought it was hunger and kept offering a feed but then DS got irritated with me - I thought he was fussing but really he just was not hungry.