The official wiki states this was never explicitly stated, but I think I know. Yotsuba. In the anime, if you look at the shoe at 17:16 in s2e2, its nearly a perfect match for what Yotsuba was wearing later in s2e4 as she chased the groper at 6:34. Not sure why she would have been out jogging that late though as she would have had no way of knowing Fuutaro & Itsuka would be taking a late night walk, but she has been shown running by the river so its not out of the picture she happened upon them and listened in.