Marshall is currently recovering from a hip injury and recently admitted that his battle to be ready in time to play in the opening match of the regular season was "frustrating" for him.
However, the 29-year-old insists that his absence from training was not related to his ongoing injury troubles and had been arranged since the start of the pre-season schedule.
"As far as me not being out there today, this is something we've had planned since camp [opened]," Marshall told ESPN.
"I had some things I needed to do, and I sat down with Coach Trestman and we decided [Wednesday] would be the best time to do it. It's nothing serious, but I'll just keep my personal business to myself."
The Bears begin the 2013 NFL regular season at home to the Cincinnati Bengals on September 8.