I heard some "slurping" noises from the crib, so I went to investigate.....
It looks like someone found her thumb! We tried the pacifier quite a few times over the last 4-5 weeks but she was having none of it, I guess she was just waiting to find her thumb! I'm not going to worry too much, we'll watch for signs of it being a problem when she's older though. I'm not surprised, she's been making sucking motions in her sleep for some time, it was just a matter of getting her hand up there.