I work not far from there and i have gone to the Erewhon a couple of times can't remember if i carried into the store or not. I generally only look at the door walking in to buildings.
The sign is legal... but good luck on seeing it. I still call bullshit, even though it's legal... Dick move my the owner.
430 ILCS 66/65 23 a-10
(d) Signs stating that the carrying of firearms is prohibited shall be clearly and conspicuously posted at the entrance of a building, premises, or real property specified in this Section as a prohibited area, unless the building or premises is a private residence.
You can store your weapon in the car
this Section shall be permitted to carry a concealed firearm on or about his or her person within a vehicle into the parking area and may store a firearm or ammunition concealed in a case within a locked vehicle or locked container out of plain view within the vehicle in the parking area. A licensee may carry a concealed firearm in the immediate area surrounding his or her vehicle within a prohibited parking lot area only for the limited purpose of storing or retrieving a firearm within the vehicle's trunk, provided the licensee ensures the concealed firearm is unloaded prior to exiting the vehicle. For purposes of this subsection, "case" includes a glove compartment or console that completely encloses the concealed firearm or ammunition, the trunk of the vehicle, or a firearm carrying box, shipping box, or other container.