Only way to make sense of it is that he wasn't following Ichigo's reiatsu... What he did was in the course of finding a SECLUDED and ALOOFED place to train, he accidently found Ichigo's reiatsu...
So two things to point out... no one passes near the training grouds where Ichigo is... and Secondly, if by any chance someone passes through there, that person probably doesn't realize that it is Ichigo that is near since that person probably doesn't know how Ichigo's reiatsu feels like...