smoking bed
n. Evidence of sexual misconduct by a politician or other public figure.
The story concerns events that may or may not have happened eight years ago, as McCain first pointed his toes toward the White House. "A female lobbyist had been turning up with him at fund-raisers, visiting his offices and accompanying him on a client's corporate jet," wrote the Times, with the breathlessness of a romance novel. Ex-McCain staffers — anonymously, natch — told the Gray Lady that "top advisers intervened to protect the candidate from himself."…Failing to find a smoking bed, the paper trashed its presumably high standards for a "gotcha" piece of journalism better suited for the birdcage.
—Andrea Peyser, “All the news fit to smear — Gray Lady's dirty tactics,” The New York Post, February 22, 2008
1991 (earliest)
Similarly, most of the press long kept rumors of Gary Hart's alleged philandering out of print, but when Hart, the front-runner for the 1988 Democratic presidential nomination, "spends the night in a townhouse with a woman not his wife, you print it because it's news," Coffey says. The press then got caught up in a feeding frenzy over Hart's sex life, though, with reporters racing each other to see who could find the next smoking bed.
—David Shaw, “Stumbling over sex in the press,” Los Angeles Times, August 18, 1991
This term is a play on the phrase smoking gun, "conclusive evidence or proof, especially of some immoral or illegal activity" (which the OED dates to 1894).