Maybe an ordinary bottle of wine at a party I'd open and pour myself a glass (if it was that kind of party), but champagne, especially good champagne, is the kind of drink that you make a big production of popping and pouring for everyone, making a toast, and savouring it. So I suspect the fact that she sneakily broke into it and drank it all shows that yes, she knew she was stealing from you. And yes, you are right to be annoyed with her for it.