Director Takeshita explained that in Yorukura first key visual, you can see Mahiru's finger as she took this picture with a film camera. That purple/darkish shadow was Mahiru's finger. #ヨルクラ
The closeup of Mahiru's expressions and eyes hint she's actually very conscious of Kano undressing behind (shown *casually* after a brief shot of her underwear). She then relaxes after realizing it was nothing like that.
A seemingly silly scene subtly communicating nice details