Wow, what wonderfully supportive comments, girls.
Cyndie, I've been sneaking food since my late teens. (I'm 40)
I would be inclined to go the non confrontational route. If you catch your daughter sneaking food, give her a big hug and let her know that you're not there to judge her, but just to be there for her. (I'm not suggesting you're judging her now... I think you know what I mean...)
My relationship with my 15 year old DD (she suffers from depression) is so tenuous in that she's a teen! I can't afford to relax my "grip" on her. Every now and then I see "that teenage look". The little rebellious one that says, "I'm slipping away. You're not on my wave-length. You're not cool, you're just my mom." And I reach for her, and hug her, and tickle her to make her laugh, and I say, "Hey! What's going on in that clever head of yours!?" And we'll sit down together and have a little chat and giggle about things, and I'll think, "Thank you, God! One more hump is behind us!"
I guess what I'm saying (in a roundabout way) is that you need to keep the lines of communication open. If your daughter feels that you're going to be judging her when she sneaks food, then she's certainly going to make a greater effort not to be caught. Not to make light of it, but even laughing together about it is very healing. We're only human, and we're going to make mistakes.
Sending you and your daughter hugs and prayers...