What does Scout find in the tree in To Kill a Mockingbird?

To Kill A Mockingbird exposes racial prejudice and stereotypical behavior as the residents of Maycomb County make countless assumptions and share vicious gossip which prevent some people, such as Tom Robinson and Boo Radley, from being treated equally and fairly. Atticus has always brought his children up to believe in the rights of others, even when others make bad decisions. He maintains that, unless  "you climb into his skin," (you) a person can have no concept of what causes others to make decisions or pass judgment themselves.

Boo Radley is a character in the novel who is much maligned and suffers because of the tales that surround him, to the point that he has been in his house for many, many years. There is apparently even a "malevolent phantom," the cause of many crimes and strange occurrences, which emanates from the Radley house. And, if the children ever knock a ball into the garden, the ball is never retrieved but is simply "lost." The children are intrigued by Boo and, prompted by Dill, they consider ways to encourage Boo to come out of the house. 

One day, whilst running home from school, Scout spots a piece of tin-foil sticking out of the knot-hole in a tree in the Radley garden. Despite her fear, she takes it and discovers that it contains two pieces of gum which she examines when she gets them home, half expecting to die after having licking one of them. On the last day of school, Scout points out another piece of tinfoil to Jem  and he takes it. This time, it is a small box, "purple velvet," and it contains coins, pennies called "Indian-heads."  

After finding a ball of twine in the knot-hole which they leave there for a few days in case it belongs to someone, the children conclude that, actually, these things are for them. The next things they find there are two small carved soap figures, a boy and a girl. A few weeks later, there was a whole pack of chewing gum and no such worries about being poisoned as the children devoured the pieces. Next, there was a medal, a spelling medal from many years before and then their greatest find, a pocket-watch. 

Unfortunately, the children will not be able to find any more treasures as, when they return to put a thank you note in the hole, they discover it has been filled with cement.