What sign? OK, that's arrogant, but the fact of the matter is that if you're doing it right, they won't know you're carrying. Don't out yourself simply because of a silly sign.
I admire your effort but have no expectation that any establishment will change their minds in our politically correct society. You made a decision to carry a gun for your personal protection. With the exception of institutions where it is absolutely illegal to do so, the worst that can possibly happen is that you are asked to leave the premises. Even if they were to engage the Police Department, as long as it's not illegal to carry in that particular establishment you're fine. I realize you aren't looking for a confrontation, especially in the era were are living through. But you're talking about your safety, which comes first.
I by-pass the "No Firearms" sign in multiple establishments where I live. No, I'm not a cowboy, a hero or a fool. I'm simply not going to be intimidated for the exact reason you pointed out in your original post, the bad guys don't care about any sign/store policy.
You have a carry permit, be responsible, intelligent and safe.
Good luck.