In the end, there were only three possibilities:
First, Sosuke Aizen had heard from Yamamoto that he was about to become the captain of the Special Operations Squad and had come to see what kind of person he was.
Second, his displayed combat strength in their previous battle had frightened Aizen and made him feel threatened, so he had come specifically to investigate.
The final possibility—and the most likely one—was that Sosuke Aizen had sensed the presence 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 Hikaru had already ruled out the first two possibilities.
That was why he had made that subtle facial expression earlier: to let Sosuke Aizen know that he did indeed possess a piece of the "Hogyoku."
He wanted Aizen to take him seriously enough not to make a move against him casually. Uchiha Hikaru had no desire to become a Visored like Shinji Hirako and Love Aikawa.
But he could not say that outright. If he did, someone as intelligent as Aizen could turn the tables on Uchiha Hikaru in minutes and put him on the opposite side of Soul Society.
That would undoubtedly place another obstacle in the path of someone who would later destroy Soul Society and lay waste to Hueco Mundo.
And most importantly...
How had Sosuke Aizen detected the aura of the "Hogyoku"?
Yamamoto Genryusai Shigekuni 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 not yet betrayed Soul Society.
Otherwise, the Espada would not have appeared only after Sosuke Aizen betrayed Soul