User blog comment:Xilinoc/Ch. 574 DEATH IN VISION/@comment-5147109-20140326144402

2 theories on Kenpachi:

1. They can't sense his reiatsu because unlocking his true potential + learning his Zanpakuto's name amped up his power to the point where his power is so high that normal Shinigami can't sense it (think Post Dangai Ichigo and Aizen)

2. His reiatsu has changed somehow to where it can't be sensed, possibly something to do with the nature of his Zanpakuto.

Guess we'll have to wait until next week to find out.