In the end, only three possibilities remained:
First, Sosuke Aizen had heard from Yamamoto that he was about to become the captain of the Special Unit and had come to see what kind of person he really was.
Second, Hiko's combat power in their previous battle had made Aizen feel threatened and fearful, so he had come to investigate.
The last, and most likely possibility, was that Sosuke Aizen had sensed the aura of the Hogyoku.
That was why he had come to investigate.
After Aizen said that he would need half a year to fully recover, Uchiha Hiko had already ruled out the first two possibilities.
That was why he had made that subtle expression—so that Sosuke Aizen would know that he truly had a Hogyoku in his possession.
He needed Aizen to take him seriously enough not to move against him carelessly. Uchiha Hiko had no desire to become a Visored like Shinji Hirako or Love Aikawa.
But he could not say it outright. If he did, someone as intelligent as Aizen could turn Uchiha Hiko into an enemy of Soul Society in minutes.
That would undoubtedly create another obstacle for someone who would later destroy Soul Society and crush Hueco Mundo.
And there was an even more crucial point.
How had Sosuke Aizen detected the aura of the Hogyoku?
Genryusai Shigekuni Yamamoto was far stronger than Aizen was now. Even Yamamoto had not sensed the Hogyoku's aura, so how had Sosuke Aizen noticed it?
It was very possible that the Hogyoku was already in Sosuke Aizen's hands, but for some reason, he had yet to betray Soul Society.
Otherwise, in the original story, the Espada had only appeared after Sosuke Aizen betrayed Soul Society.
They had not appeared when Ichigo Kurosaki had only just become a Soul