report suspicious behavior

The city of Chicago has a warning posted on their official website.

Be sure to call 911 (in Chicago) if you see any "note taking, binocular use, cameras, videos, maps" around the holidays. You never can trust someone that's out taking notes and pictures in public spaces, especially this time of year. If you're clever you could take notes on the suspicious behavior, maybe take their picture, or get a better look at the suspicious person with binoculars, or shoot video of them, then mark down their location on a map. But then you might get arrested for, well, behaving suspiciously.

The full URL is:
http://webapps.cityofchicago.org/ChicagoAlertWeb/resources/pdf/WinterHolidayPublicAwareness.pdf

But you can also get to it from here.