He must've known that Kisuke Urahara put it in Rukia's body and partially knew why she has been out in the real world for so long Aizen assumed something was up and wanted her to be executed, knew people would come to rescue her and then at that moment he would take advantage of the Ryoka's and keep them alive and have them save her just for him to take the thingy. lol Aizen had an idea from the start ever since Urahara was a captain.
lawl some of the posts here are funny anyway I kinda think based on urahara's research and his general knowledge on urahara's personality he probably estimated that urahara had put the orb inside rukia that's just my crazy logic that noone really cares aboutX3