The superstar receiver wasn't on the field with teammates for Tuesday's voluntary session, as NFL Network's MJ Acosta reported.
Brown had been a participant in previous portions of the team's voluntary offseason workouts.
"He's not here today; hopefully, we'll see him in the next couple days," Raiders head coach Jon Gruden told reporters Tuesday. "He's been working extremely hard learning our offense and really excited to get him out here, but in the meantime we have plenty of balls to throw and plenty of receivers to throw to."
Plenty of players -- from Tom Brady to Julio Jones to Le'Veon Bell, etc. -- exercise their right to skip voluntary workouts, even as clubs move into team-drill portions of workouts with the start of OTAs. Given Brown's offseason, which brought a battle with the Pittsburgh Steelers, a trade to Oakland and a new contract, his absence from voluntary sessions is notable.