The box that was empty, had a 1979 on it. Shioi would have been about 5 years old then. Could it be that Shioi originally had the ability to see "them" but she was severed from it using a sacrificial corpus? In the chapter where they severed her collegue's sight, the exorcist first ask about for dolls that he held dear, implying that those would make a perfect corpus. What if they made young Shioi use her favourite doll for a corpus? Then that doll would be a replacement they used to console her.
It would make sense if she couldn't remember given she was so little and had to "kill" her favourite doll. That would also explain while she looked cursed to that one Wind God in an earlier chapter. Her days were numbered even before meeting Asama
