"Where the Joker's involved, we hedge our bets." Batman unclipped a small device from his Utility Belt and turned it on. It was a portable X-Ray machine customized to detect the presence of most common explosives and toxins, and even radioactivity. He held it out over the envelope, and an image appeared on its screen.
 
"It's a USB drive," he said. "There doesn't appear to be anything else inside."

Batman: Arkham Knight: The Riddler's Gambit, Chapter 2, Pg. 21