The spokesman said both are there primarily to visit David Cutcliffe, who was Tennessee's offensive coordinator when both were college quarterbacks.
The cover was blown earlier Thursday when Matt Skura, a Duke football player, posted on Twitter that he was "stretching" next to Manning. The post was later taken down.
Manning has spent the past five months recovering from his third neck surgery in less than two years, and there have been conflicting reports about how much progress the four-time league MVP has made. Manning's doctor has cleared him to resume playing, although Colts owner Jim Irsay says he has not yet passed the team's physical.