Gandalf stays with Frodo for a while in The Fellowship of the Ring after Bilbo takes off. Then one day he takes off, saying he’d heard something “worrying.” What did he hear about?
Foreboding of danger:
At the end of June I was in the Shire, but a cloud of anxiety was on my mind, and I rode to the southern borders of the little land; for I had a foreboding of some danger, still hidden from me but drawing near. There messages reached me telling me of war and defeat in Gondor, and when I heard of the Black Shadow a chill smote my heart. But I found nothing save a few fugitives from the South; yet it seemed to me that on them sat a fear of which they would not speak. I turned then east and north and journeyed along the Greenway; and not far from Bree I came upon a traveller sitting on a bank beside the road with his grazing horse beside him. It was Radagast the Brown, who at one time dwelt at Rhosgobel, near the borders of Mirkwood. He is one of my order, but I had not seen him for many a year.