I caught that the officer told MT that in the interview, but I did not see that visually on the video. To me, it appeared that, after dropping something white into the garbage bin at the curb behind the Raptor, FD strolled over to the storm drain opening beside MT's open passenger door and tucked the envelope into that opening.
Either LE has access to a clearer video than we saw OR they were saying that to see if she'd correct them. (Or, possibly to plant the fear that LE might have her prints on the envelope...?)