Perhaps you've heard of constantly circling Cessna planes, spending hours circling high above US cities. http://www.washingtontimes.com/news/2015/jun/18/fbi-surveillance-flights-by-the-book-rarely-track-/?page=all Today I happened to see many of them over Chicago. Looking at FlightRadar24's live feed I saw two. Their data is much more complete than my limited ADS-B receiving capability. The first was a Cessna, high at 9,000ft. It looks like it came from DuPage County airport, and has done a number of similar flights in the past few days. I was able to spot it in binoculars, and in doing so also picked up an identical looking plane holding an identical path, but not on ADS-B. I presume I was seeing a shift change, but one never knows. It had the 4414 squawk, and matched the false front company 'OBR Leasing'. Not only Cessnas, but a Bell 407 helicopter was hanging around too at a lower altitude further west, closer to Midway.