-he searched every damn where for the ricin, then walt shows up and whaddaya know, it turns up somewhere jesse already looked
-when he's at the pick up point, he realizes that there's been a switch and Huell took something out of his pocket
-he then realizes that this has happened before and how he lost the ricin in the first place
I think? All of that could be completely wrong, but I think it's something like that.
no, that's it. the big thing was huell. he takes the joint from jesse. he could've just left it alone jesse probably never would've put it together that huell lifted it (the ricin cig) the first time.
looking back the only thing that felt overly contrived to me was jesse lighting up in saul's waiting room and office. it was a little out of character at the time. but it was a device that was needed in order to have a reason for huell to try to lift it and hence connect the whole ricin incident with brock.