[The Might]

I really doubt it was Saruman, as no matter how much he feared them, I doubt it would be too much for him to not try to get the Ring. It was the object of his desires and so if he thought the Three Hunters may have it, he would have surely attacked. Of course, he may have sensed that the Ring was not near and so decided to make an exit.

A phantom also seems somewhat unlikely, although if Saruman had indeed studied the dark arts he may have learned something on this topic. Maybe he just looked into the palantir and thought "ah well, could at least try to annoy them with a spooky ghost".

What I now actually find most plausible of all is Radagast being there. The only problem is - what would Radagast be doing there? If he had indeed come to Fangorn, why didn't either Gandalf nor Treebeard mention him? At least Treebeard should probably know.
But it does somewaht make sense - Gandalf informed Radagast to tell all the birds and beasts in Mirkwood to round up information on the actions of the Enemy on Midsummer's Day. On September 18 Gwaihir bearing news to Isengard found Gandalf imprisoned. In that time Radagast went to Mirkwood, probably to Rhosgobel and sent out the word and from there it wasn't that far to the eaves of Mirkwood. Especially by taking the Redhorn route he could have made it that far.

The only problem I have is - after leaving Gandalf why would Radagast make his way towards Isengard instead of chilling in the forest with birds and beasts? It was not really his character to do that.