"There are people and things in our universe which I know only by their effects," Edric said, his fish mouth held in a thin line. "I know they have been here... there... somewhere. As water creatures stir up the currents in their passage, so the prescient stir up Time. I have seen where your husband has been; never have I seen him nor the people who truly share his aims and loyalties. This is the concealment which an adept gives to those who are his."
Now, only people with the Siona gene are permitted free movement on Chapterhouse planet, so that no prescient could track its location. However, if prescience-shielded individuals still leave traces that can be seen, doesn't this render the precaution useless? Wouldn't a Navigator just need to look for places where weird things happen like food is cooked and then eaten all by itself ( ) etc.? Or does such a high concentration of humans with Siona gene somehow make the entire planet invisible? Maybe the "cloaking effect" can spread to the environment as well (just like a prescient person can also hide those who share their loyalties)?