Um...she literally posted a picture of the hoodie and tambourine. Furthermore, as was pointed out above, the film didn't come out for another three years and was quickly forgotten outside of diehard fans. It's not unreasonable to say that she didn't attend the screening for a variety of reasons. Furthermore, even when she did find out she was in the film, that doesn't mean she would necessarily run to the nearest Zeppelin forum to see people speculating on who she is. She attended a concert, got her face on film for 10 seconds, and that was it. She probably thought "cool, they caught me on film!" and didn't think more of it. I don't really think it's that hard to believe.