That varies by state. In Michigan, the sign is a "suggestion" just like you said for your state. In North Carolina, the sign had the force of law and if caught carrying you lose your permit.Not 100% sure on the laws in Indiana, but in my state a private business just putting a sign up that says 'no guns' does not make it illegal for a permit holder to carry a gun into those buildings. If they catch wind that you have a gun they can choose to trespass you and then you can't return by law. A lot of people don't understand that. They think the sign is magically a law that blocks guns from entering. Morons.
But then again, if you are concealing properly no one would ever know anyway so it would be hard to trespass you.