Chicago helicopters that have their transponder turned off

• The very useful list of helicopters

Random discussion threads that might be relevant

• https://dailynorthwestern.com/2025/11/07/top-stories/when-federal-immigration-agencies-came-to-evanston-their-helicopters-went-off-the-grid/
• https://www.reddit.com/r/chicago/comments/1ok3dre/helicopter_circling_avondaleoip_area_not_showing/