After intellectually understanding the truth that one is the “Knower”, we start looking, checking, comparing whether the body-mind is thinking, feeling and behaving as per that intellectual understanding.
Why that preoccupation whether one is “there” or not arises?
It arises due to the simple mistake of forgetting the intellectual understanding that you are not the body-mind and slipping back into the body-mind as “me”.
You think a particular body-mind is “me” and some other body-mind is not “me” and so what is happening in or to that body-mind which you do not identify with as “me” does not affect you in any manner, unless it be related in some fashion to the particular body-mind you are taking to be “me”.
Hence, understand this second layer of ignorance if it supervenes on the earlier intellectual understanding.
Think.