They can't publicly show it but fans know.
Apparently, Tiffany played Jessica's music during her concert tour, Sooyoung and Jessica share many mutual friends and attend the same parties.
For Yoona, she's the only member who openly shared a photo of 9 on their 10th anniversary and the two were best friends.
It is simple for the fans to spot them together if they really hanged out especially on the areas where they had to go at the same places for events. They've should've been spotted before. That's just me