I definitely think it's a little rude. It would bother me solely for the fact that she was taking things without asking first. Like you said, it wouldn't be such a big deal if she just ASKED. And even if she doesn't ask, she should take in moderation..not eat the entire box of granola bars or eat the majority of the cookies.

Maybe she didn't offer an apology or an explanation because she was embarrassed about the whole thing, or maybe she just doesn't find it to be that big of a deal.

Either way, you've expressed that it bothers you, now all you can do is wait it out and if she continues to do it, thats when you snap!

Good luck!!