This sounds like a plot for a bad conspiracy movie, but here it is printed in the Lodi CA newspaper:
Mystery planes continue to circle over Lodi (By Layla Bohm, News-Sentinel Staff Writer, Last updated: Wednesday, Jun 29, 2005)
The story is part of a larger one, concerning some locals who tried to get training in "terrorist training camps". The FBI came to town to interrogate some people, and at the same time these small airplanes began flying in circles all day long. It's a small enough town that people took notice, and have been investigating on their own.
"He's doing something. He's doing some reconnaissance," said Lodi resident and pilot Arlene Farley, who even got out binoculars to peer up at one of the planes.
What the planes are doing remains a mystery, though most people believe the activity coincides with the FBI investigation that led to the arrests of five Lodi men. In other parts of the country, small planes have flown in circles over cities also under investigation on ties to domestic and international terrorism.
What are their clues?
First is the registry of the mystery plane. It's registered to a shell company in Delaware, Northwest Aircraft Leasing Corp., with anonymous owners. If you enter that name in Delaware's Secretary of State Corporate Information lookup, you find the corporation is registered by the THE CORPORATION TRUST COMPANY, a company that specializes in helping out-of-state people register their corporations in Delaware (because Delaware doesn't tax corporations).
The second clue is that further queries about the plane, using the tail number, keep turning up people who refuse to say any more. On initial contact with the FAA, a spokesman there supposedly suggested the airplane was likely recording cell phone conversations and tracking people with infrared. The FBI, of course, says they cannot comment on existing investigations.
Recall that the CIA has a history of getting airplanes registered under dummy corporations (e.g. the famous Air America that operated in Vietnam) that it then used for nefarious purposes. One such story appeared in May 2005 when NY Times reporters investigated airplanes being used for "extraordinary rendition" cases where the CIA is outsourcing torture to countries that have fewer qualms. The CIA has all sorts of use for airplanes, not just shipping prisoners around, and apparently this is one of them.
So far these two clues aren't much. The most important clue is the airplanes behavior.
It is flying continuous circles to the left (by which, I think they mean counterclockwise). This behavior is recommended by FLIR Systems, makers of a thermal camera (infrared) that's meant to be used by law enforcement in conducting aerial searches. The March 2003 issue of their magazine, Field of View, describes the procedure and recommends using the Cessna 182 because of its low noise signature.
None of which really answers the concern ... it all simply is information that makes one go "hmmm"