Hi @Talsip !
Illinois taxes non-residents on money from Illinois sources. You will have to file an Illinois return if your taxable income exceeds the exemption amount of $2,625, if you are filing as a single person. You will also want to file to claim a refund if your employer withholds Illinois taxes in excess of what you owe. You can check with your employer and make sure they are withholding and paying Illinois tax on your behalf.
Hope this helps!
Cindy