A police officer is not going to send you a ticket in the mail. They, or a colleague, are going to pull you over there and then.
Photo radar vans in Illinois look like this:
The places to watch out the most right now are in work zones on the Edens, on the Tri-State Tollway (I-294), in Will County and near Champaign. Here's a good video link from a while ago about how they work. CBS2-Chicago video library
If one of these vans catches you speeding in a construction zone in IL, the photo radar ticket is a minimum first-time fine of $375. A second fine will be $1,000 plus a suspension of your drivers' license for 90 days.
If you don't mind an impartial opinion on your radar detector, I personally wouldn't feel comfortable relying on an S2 with those kind of fines flying around. Sure, it's the best cordless detector but compared to even mid-range or low-end "Cord-powered" units, its performance is severly compromised, especially on K Band, which is what you're up against here. I'd recommend you research a bit more on alternative RD's - ask some of the guys here for recommendations. You'll be glad you did.
For reference, red light camera fines in Illinois can be up to $200 (Max allowed by State statute) but most cities stick to $100.