IL requires a Licensee-In-Charge (owner of security company) that is registered with the IL Dept. of Professional Regulations. A Security Officer can at the direction of a 'LIC' get the IL Armed Security Licence (Tan Card, FAC, Whatever they are calling it). As long as the Officer is working they can carry a firearm they qualify with. There are 4 classifications of Firearms. Revolver, Semi-Auto Pistol, Shotgun and Rifle. The Security Officer at the direction of the LIC can carry plain clothes and on duty. I've not been in Security in IL for a number of years.
This is how PI's, Process Servers, and VIP Protection details carry in IL in plain clothed concealed.
Now that is a goofy loophole in the law, and THE ONLY way a non police officer can carry concealed in IL. (so far)
What dipwad did at the airport, and his buddies, is abusing that law. Citing that he just got off duty and forgot it? Really? That should get his 'golden' security lic. pulled with jail time, not to mention getting time for the stunt at the airport. He still has to follow the 1 hour to and 1 hour from work rule.