Atticus broke up Jem's attempt to send a note to Boo Radley via a fishing pole in Chapter 5. Jem was trying to thank Boo for the gifts the children had been left in the knothole of the tree.
"Son," he said to Jem, "I'm going to tell you something and tell you one time; stop tormenting that man. That goes for the other two of you."
But Jem was not through with his attempts to communicate with Boo. On Dill's final night in Maycomb in Chapter 6, the children decided to try once again. Although they didn't carry a note with them, they decided to try and get a peak at Boo through the Radley porch window. But when a mysterious shadow suddenly appeared, the children ran for their lives. In the process, Jem lost his pants in the barbed wire fence and a shotgun blast rang overhead. Only Dill's quick thinking saved Jem from more trouble. When Atticus questioned Jem about why he was standing in public in his underwear, Dill responded:
"Ah--I won 'em from him," he said vaguely.
"Won them? How?"
Dill's hand sought the back of his head. He brought it forward and across his forehead. "We were playin' strip poker up yonder by the fishpool."
As he got scared away, he thought he might have seen a slight bit of movement in the window, like the draperies had moved or something. This led him to believe someone was watching him.
At a different point, when Scout rolled into the tire and landed in the Radley's front yard, she heard someone laughing.
Both of these are signs of life for Boo Radley to the kids. It still leaves some question and suspense, but they point to a future friendly relationship between the phantom and kids.