Fanny packs are targets. Period. They are not worth the trouble - actually, they bring the trouble. I would never take one.
Instead, we take with us a backpack that holds our touring day supplies. One with zippered compartments. Then we lock the compartments with those little locks you can buy on-line or at a good travel store that have a three digit combination. This also makes you appear local, as many people in Europe carry them, and since we have both our camera and video camera in it, I can attest that access when needed never seems to be an issue. We take the cameras out when at a site we want to film and I leave the video on a strap around my neck.
We keep all kinds of other items in the backpack we may need on touring days, such as windbreakers, kleenex, medications. Since it seems to always be part of you anyway, no one grabs at it either.
I know for a fact that thieves can get into fanny packs without the owner knowing it. If you insist on taking one, they also sell smaller fanny pack locks, but they can be hard to see down there on your hip to work the combination.