The advaitins are clever about negation. First of all there is the concept of superimposition or the mis-take i.e. you took something to be that which it was not. That such error is also the explanatory analogy for a theory of knowledge that slips between the Scylla and Charybdis of Realism and Idealism is a cunning lateral swerve. That requires deep explication but let it stand for now.
Next there is the unknown object, the ajnanatta satta. That an object can exist as an unknown object is an indication of its reality. It shelters under the great umbrella of being even if covered by a fog of ignorance. Mental existence is only for the time in which its object is present to the mind. It only exists by being known - it cannot be an unknown existent.
Finally there is the means of valid knowledge known as anupalabadhi or non-apprehension of existence. The book that was supposed to be on the table in the hall is not there. My friend was not in the cafe where I expected him to be. Please do not confuse this with the apprehension of non-existence which is impossible because that which does not exist cannot be apprehended.
Addendum: previously snakes and ropes