Jem finds Dill hiding under the bed.
In chapter 14 when Scout and Jem are sent to bed early for fighting, Scout goes in to her room and steps on something "warm, resilient, and rather smooth" (Lee, chapter 14). Scout assumes that it is a snake and calls Jem to help her. He gets a broom and jabs the broom stick under the bed. After hearing grunts, Jem sees a dirty person run out from under the bed. He swings the broom and luckily misses because it is Dill.
Dill has run away from home because he feels mistreated and he also misses Scout and Jem.