PP 117-118 of "The Answer"
[spoiler]A Humvee containing Tom, another Human-Controller as the driver, a Cassie faked up by Erek, and several Animorphs in insect form drives up to the wall of Hork-Bajir who are protecting the Pool ship. A Hork-Bajir and a Human-Controller approach to do an inspection. The Human-Controller introduces a sensor probe, which reads only two complex life forms (Tom and the driver) and the insects. It doesn't read Erek-Cassie as a life form, but Tom rebuffs the guard by asking him if he can count to 3. Why doesn't the senor probe detect the Yeerks inside Tom and the driver? [/spoiler]