Britney Spears was in court yesterday for another review of her conservatorship, and E! says that, though she appeared to be "in good spirits", she wasn't wearing her engagement ring. And she wasn't wearing it 5 days ago in Long Beach either. In fact the last time she was photographed with it was almost 2 weeks ago.
This can only mean one thing; that she lost it about two weeks ago. Probably during some sort of pig chasing accident, or while she set a puppet on fire because it started talking to her during a show and scared her.