At the end of Breaking Bad Season 4, we learn that
Gus had never poisoned Brock at all. It was Walt's doing using lily of the valley; a way of manipulating Jesse into helping him murder Gus.
But if this was the case, where did the ricin go? Walt had already convinced Jesse that he could not have taken the ricin as Jesse had it with him in the morning and the only people who had any sort of access to it afterwards were Jesse himself and possibly Tyrus.
But Walt's plan hinged upon Jesse believing that Gus had stolen the ricin and used it on Brock. So Jesse couldn't have just lost it.
How could Walt have stolen the ricin? The most likely explanation IMO is that Walt stole the ricin during the three day period before the symptoms are supposed to kick in. That way Jesse wouldn't suspect him for lifting the ricin (as he would only look for any opportunities Walt may have had before three days prior to Brock's sickness). But I can't think of any opportunity that Walt had.
So, how could Walt have stolen the ricin?