Project Daylight was a secret group project between the White House, CIA and the FBI agencies. It gathered and distributed illegal NSA information for the government using illegal intel and fake informants to fake convictions that should never have stood.
After Guerrero was apprehended by Kurt Weller and his team, at interrogation, Guerrero didn’t recognize Mayfair and Weller realized that Guerrero wasn't Mayfair's informant as she previously claimed; it was then that Mayfair disclosed the truth about Daylight to him.
Prior to his death, Thomas Carter expressed great concern that Jane Doe may know something about Project Daylight, something he went to extremes to find out and tried to convince Mayfair of killing Jane in order to prevent her of saying anything about the operations.
Following Sandstorm successfully framing Mayfair for Carter's death, Weller visited her while she was under house arrest on bail and insisted that she come forward with the truth about Daylight to acquit herself. Mayfair refused, knowing that all the criminals she put away with Daylight's intel would be free as a result.
Before her death at Oscar's hands, Mayfair left a USB drive with full information on Daylight for Weller's team.