The 07 Chicago code does not specifically require a people door on a garage, that I can find. The closest applicable section I could find is 4(13-96-280).
If this is a new garage it was built without a permit or not to permit. I work with a garage builder doing inspections and know that they cannot get a permit through DCAP without a people door. You can check the DCAP website for garage permit info and it should be listed there.
IRC does not apply to Chicago. Any challenges you make have to be to Chicago code or such challenges will not have traction.
Good luck

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Accurate Inspections & Consulting, Inc.
773/844-4AIC
Markus Keller