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”
(”The Riders of Rohan”)
The old man stays silent and as Aragorn tried to approach him he was gone without any traces.
Gimli is quick to state that it is Saruman, but then Aragorn says “this old man had a hat not a hood” – unlike Saruman’s usual hooded appearance.
In the following chapters, when the three meet Gandalf, Gimli asks him if it was him (Gandalf) or Saruman that they saw to which Gandalf replies:
‘You certainly did not see me,therefore I must guess that you saw Saruman. ”
So, firstly. This old man had a hat and not a hood, and while it is not impossible but I find it unlikely for Saruman to be "hatted".
And then, why would Saruman be interested in three members of the Fellowship, who had nothing to do with the One Ring directly – unlike hobbits? Especially when he had more important plans a.k.a. making war on Rohan?
So, if it was not Gandalf and not Saruman, the only other option I find is Radagast.
Of course, I do not believe Tolkien ever said anything on this or even had this in mind when he wrote the chapter but I find it an interesting speculation. What are the counter-arguments for this, then?