It’s NEVER about the seatbelt. They use that to stop you, and once you’re stopped they look in your car (and smell you and your car) to see if they can find anything else. The traffic stop is the gateway. (They admit as much).
(Full Disclosure, I watch a lot of Cops, and I’m not anti-cop).
I’m retired Law Enforcement. I never worked in a Big City, so this may not be transferable information, but where I worked (Rural & Small Cities) we were not allowed to do traffic enforcement unless in uniform. Perhaps Chicago does things differently.
I thought Chicago passed some pretty crazy regulations for Police/public interactions.
I thought:
No One was to be pulled over for any violations such as:
Tail Lights
Registration sticker
seat belt
headlight
etc...