When Aragorn, Gimli and Legolas find the dead bodies of the orcs that they had pursued to the edge of Fangorn, they see an old man robed in white:
Suddenly Gimli looked up, and there just on the edge of the fire-light stood an old bent man, leaning on a staff, and wrapped in a great cloak; his wide-brimmed hat was pulled down over his eyes. Gimli sprang up, too amazed for the moment to cry out, thought at once the thought flashed into his mind that Saruman had caught them
It is implied in the book (though never verified AFAIK) that this was Saruman, particularly since Gandalf confirms that it was not him
"You certainly did not see me," answered Gandalf, "therefore I must guess that you saw Saruman."
So if it was indeed Saruman, why would he travel all the way to Fangorn alone? Orthanc to Fangorn is not a short journey and he also had the Palantir at his disposal so what is the purpose of his being there (assuming that it is indeed Saruman).