If I remember correctly, the EYE served as a hint to Kazuha and her friends that the reason why the police cannot find the murderer is because of the special tiles with the crazy diamond optical illusion. Remember the case had two culprits, the last culprit was hoping for someone to solve the murder but no one did it in time so she took the matters to her own hands and revenged. It could have a double meaning, but honestly I think it is unlikely that Sakurako is Rum for the same reasons the posters above have said. She's so young and seems to have known Naeko for a long time. And housemaid is a sad disguise for rum T_T And Rum, being a strong distilled liquor, will most likely be a male.
On a side note, the case is also where Koumei reappeared as a recurring chara (he got transferred back to Nagano HQ). I doubt he's Rum because he's known Yamato and Yui for so long. He doesn't seem evil either...