User blog comment:Xilinoc/Ch. 581 - THE HERO 2/@comment-24153155-20140521225952/@comment-3250274-20140522164209

Regarding the reiatsu of the Shinigami returning from the palace not being detected; remember Aizen spoke of reiatsu reaching a certain level that it cannot be detected by those with less, and only by those who are on that level. Maybe they are just at that high of a level now. Perhaps the clothing they were is also reiatsu concealing like Kisuke's cloak.

"After Aizen fuses with the Hōgyoku, both Shinigami and Humans are incapable of sensing his Reiatsu unless he voluntarily lowers it and allows them to interfere. He compares this to how a two-dimensional being is incapable of interfering with a three-dimensional one.[211] Isshin surmises Aizen's Reiryoku can only be sensed by someone who is at the same level as him."