A surveillance program that monitors Americans on domestic flights, even if they are not suspected of a crime or having ties to terrorism, is being questioned by civil liberties advocates.

"The whole thing is just absurd on so many levels," said Hugh Handeyside, senior staff attorney with the American Civil Liberties Union's National Security Project.

The program — dubbed "Quiet Skies" by the Transportation Security Administration — has been in existence since 2010 but was disclosed for the first time this past weekend by The Boston Globe.

The Globe said "Quiet Skies" tracks U.S. citizens who have been flagged to the TSA based on their affiliations or travel histories. One businesswoman who had recently traveled to Turkey, for example, was tracked.

If a passenger is selected for such secret tracking, a federal air marshal monitors him or her during the flight. The air marshal notes in a "behavior checklist" whether the individual slept, shaved or changed clothes mid-flight, or boarded last, among other criteria. The air marshal also takes note of whether the passenger has a "cold penetrating stare" or is fidgeting, the Globe reported.

The data is then sent to the TSA, although it's not clear what happens to the information afterwards.

In a statement to NBC News, the TSA described "Quiet Skies" as a "practical method of keeping another act of terrorism from occurring at 30,000 feet." It compared it to other common practices in law enforcement, like stationing a police officer in an area vulnerable to crime.