When Dusty puts his right arm around Doris during their lunch-time rendezvous, the DDT or sulfur powder on his hand leaves a faint, but visible, hand print on the back of her dress. Back at her office, when Mathews notices the mark, it has become a large, very noticeable light-colored smudge that covers nearly half her upper back. [The hand print could have smeared when her back touched the seat back in her car.]