Trestman confirmed that Marshall had always been due to miss today's session, but would not delve any deeper into why he was absent other than attributing it to personal reasons.
"This was a planned-out miss today for personal reasons," Trestman told reporters.
"We wanted to let him get through practice [on Wednesday], but we let him go for personal reasons. It's something that was part of the off-season plan since we started training camp.
"I'd rather just leave it to personal reasons out of respect to Brandon, but it's all good. As I've said when I let guys go for personal reasons, it's all good, and this is all good. It's not reflected in anything other than I gave him time off."
Marshall will miss his side's final warm-up game against the Cleveland Browns on Thursday but is hopeful of playing against the Cincinnati Bengals on September 8.