There's something that's puzzled me ever since we learned of Yhwach using Auswählen on the impure Quincy to regain his power. Based on what was stated earlier in the manga, following the Quincy genocide 200 years ago, the surviving Quincy were supposedly monitored closely even as their numbers continued to decline. There's no exact indication as to just how many there were at the time, but there had to have been quite a few left for Auswählen to restore Yhwach's power. That being said, even if the ratio of Impure Quincy to the number of Pure Quincy was relatively low, if the Soul Society was really monitoring the Quincy as much as seemed to be implied, it would seem unlikely that the near-simultaneous deaths of so many Quincy as a result of Auswählen would have gone unnoticed by the Soul Society, and would have alerted those in the know, like Yamamoto, that Yhwach was the most likely culprit, and would have made finding him a top priority (not that they would have, since he and his followers were hiding in the Soul Society all along, the last place the Shimigami would think to look). Can anybody think of a reasonable explanation as to how they seemed to fail to notice?