I'm wondering if grain was found on their bodies or on their clothes.
I sort of believe if it were found on their bodies that silo would have been searched sooner. But clothes and other evidence they have been steadily going back to the CS to gather would take some time to analyze and they may have discovered it. Other posters have said it's not like the crime shows we watch where discoveries like grain are all made in the first 30 minutes. Plus there is the backlog as previously noted. Seems like things sure changed in this investigation after they searched around that silo last Friday.