I bought a fanny pack for guns and found it to be far too floppy for any amount of heavy weight, so primarily, I use it backpacking. It is really nice to be able to have everything outside of your pockets, because especially in winter backpacking, you are layering and delayering your pants.
100% of the people who I have known to wear fanny packs had no guns in them. I would suspect most people don't have guns in their fanny packs across the country. Concealed carry people are pretty rare. Lots of people sign up for a concealed carry licence, then never carry. Just think, carrying a gun in a fanny pack makes you an extremely rarified individual, like <0.1% of the population.
Further, most of the people who I have seen wear fanny packs, did not use them backpacking either. Hiking. Festivals. That sort of thing. I think you could get away with urban carry, especially if it looked like a camera case. Nobody thinks fanny packs are cool except for the people carrying them, so you may look like an odd ball, but I wouldn't assume they carry, like others have said, except for concealed carry types, nobody else will either.
That said if you have 5.11 or Maxpedition on your fanny pack, then people might more rightly assume that you have a gun in there. I've approached a woman, because I like Maxpedition Gear, and she DID carry in hers but she wasn't carrying when I spoke with her about her pack. She had been harassed (also while not legally carrying) because the bouncers at some event knew that Maxpedition products were often designed with carry in mind.
That said, I've never been harassed while carry or not carrying (95% of the time) my Maxpedition fanny pack.