According to Pekin Police Chief Greg Nelson (received via email):
Business owners are able to set workplace policies and post requirements for admittance into their businesses. Very similar to “No Smoking”, “No Pets” or “No Shirt, No Shoes, No Service” signs, businesses that wish to bar people from bringing firearms into their businesses should post “No Firearms” signs. If officers are called to a place of business that has someone on their property that is carrying a firearm and will not leave, officers should instruct the person to leave. If the person refuses, they would be arrested for Trespass, NOT for a firearms violation.
If an employer has an employee that is in violation of a workplace policy regarding carrying firearms, the employee would be subject to action from their employer for a policy violation. We (Pekin Police) normally would not get involved unless the employee is ordered to leave the property and refuses to, at which point we are again dealing with a trespasser.
Until the Governor signs the bill, we are without bright lines to work with. Use good judgment with each of these situations and be sure to document not only what you did, but why you did it. I plan to share this with the Chamber of Commerce, City Council and City Staff Members so that we are all on the same page on this issue.
Once Governor Quinn takes action on the legislation that has passed the House and Senate, these gray lines should be made clearer.