He already suspected Huell of lifting the ricin cig from him in the office. Walt had to convince him otherwise. Huell lifted his dro on Saul's orders, that's what he's searching for while he waits for the disappearer. The cigarette packet triggers relational memory; you have to remember that in the Breaking Bad timeline the Brock health scare only happened a few months ago. From there Jesse connects the dots. From there Saul spills the beans on Walt endangering Jesse's surrogate son in order to turn him against Gus.