When I read your "A little about me" section, my first thought was that your daughter is testing some boundaries due to her new found "freedom" being in public school. Not that being in public school is a bad thing...She's just got a taste of something new and is spreading her wings a bit.
I know that having a sneaky child, in any aspect, is worriesome and it makes you wonder what else they might be hiding, however I think sneaking a granola bar is probably not something to be too concerned about.
I remember when I was a child, probably the same age as your daughter, I broke a glass while dusting and told my mom it wasn't me, even though it was pretty obvious it was. I think my mom did the best thing we can do as parents by letting me know that it was OK if I broke something, etc. but I needed to be up front with it and tell her. That opens a line of communication between children and parents, which will be needed in the years to come.
Good luck and don't be too hard on her for these types of things. Just let her know you're there for her, but she must respect the rules.