After Joy Birch mentions Keller's name at the hospital in Prisoners, Keller rushes off as Loki chases him in the hospital, then to a car chase outside. Loki loses Keller.
So why did Joy Birch mention Keller's name at hospital?
As we find out at the end of Prisoners, Holly was the one who kidnapped Joy and Anna. Right before Joy escaped, Keller had visited Holly's to apologize for how he treated her nephew Alex. They talked for a bit and Keller left. When Joy sees Keller at the hospital, she remembers him being at Holly's house. Joy says "You were there, they put tape on my mouth." Meaning that when Keller was at Holly's house, Holly put tape on the girl's mouths to keep them quiet. It takes Keller a few seconds to realize what Joy meant and when he did, he takes off to Holly's house to save his daughter.