A Constructive Survey Of Upanishadic Philosophy: Being An Introduction To The Thought Of The Upanishads by R.D. Ranade. This is from p 163-165. Here is a link.
As we have said, we shall examine the ideas instead of words in the Upanishads, and see whether the traces of Maya doctrine cannot be found in them. The Isopanishad tells us that truth is veiled in this universe by a vessel of gold, and it invokes the grace of God to lift up the golden vessel and allow the truth to be seen [Is. 15]. The veil that covers the truth is here described as golden, as being so rich, gaudy, and dazzling that it takes away the mind of the observer from the inner contents, the rivets it upon itself. Let us not be dazzled by the appearance of gold, says the Upanishad, everything that glitters is not gold. Let us penetrate deeper and see the reality that lies ensconced in it. We have thus, first, the conception of a veil which prevents the truth from being seen at first glance. Then, again, we have another image in the Kathopanishad of how people living in ignorance, and thinking themselves to be wise, move about wandering, like blind men following the blind, in search of reality, which they would have easily seen had they lodged themselves in knowledge instead of ignorance [Ka 1.2(4,5)]. We have here the conception of blindfoldness, and we are told that we deliberately shut our eyes to the truth before us. Then, thirdly, ignorance is compared in the Mundakopanishad to a knot which a man has to untie before he gets possesstion of the Self in the recess of his heart [Mu. 2.1.10]. Fourthly, the Chandogyopanishad tells us how knowledge is power, and ignorance is impotence [Ch. 1.1.10]. We, who are moving in this world without having attained to the knowledge of the Atman, are exhibiting at every stage the power of impotence that lies in us. Not unless we have attained to the knowledge of Atman can we be said to have attained power. Then, fifthly, the famous prayer in the Brihadaranyaka, in which a devotee is praying to God to carry him from Not-Being to Being, from Darkness to Light, from Death to Immortality, merely voices the sentiment of the spiritual aspirant who wishes to rid himself of the power of Evil over him. Unreality is here compared to Non-being, to Darkness, or to Death [Br. 1.3.28]. The Kathopanishad declares that Sages never find reality and certainty in the unrealities and uncertainties of this world [Ka. 2.4.2]. Maya is described as an adhurava -- an unreality, or an Uncertainty. The Changogya again tells us that a cover of Untruth hides the ultimate Truth from us, just as the surface of the earth hides from us the golden treasure that is hidden inside it. We, who unconsciously move to the region of Truth, day after day, do yet labour under the power of Untruth for we do not know the Atman. This Atman is verily inside the hearts. It is only he, who reaches Him everyday, that is able to transcend the phenomenal world [Ch. 8.3(1-3)]. Maya is here compared to an untruth, an "anrita". Then again, the Prashnopanishad tells us that we cannot reach the world of Brahman unless we have shaken off the crookedness in us, the falsehood in us, the illusion (Maya) in us [Pr. 1.16]. It is important to remember that the word Maya is directly used in this passage, and almost in the sense of illusion. In the same sense is the word maya used in Svetasvarata where we are told that it only by meditation upon God, by union with Him, and by entering into He being, that at the end there there is the cessation of the great world-illusion [Sh. 1.10]. Here again, as before, the word Maya can mean nothing but ilusion. It must be remembered, however, that the word Maya was used so far back as at the time of the Rigveda in a passage, which is quoted by the Brihadaranyaka, where Indra is declared to have assumed many shapes by his Maya [Br. 2.5.19 and RV. 6.47.18]. There apparently, the word Maya meant "power" instead of "illusion" -- a sense in which Shvetashvatara later uses it, when it describes its God as a Mayin, a magician, a powerful Being who creates this world by his powers while the other, namely, the individual soul is bound again by Maya [Sh. 4.9]. Here is must be remembered that there is yet no distinction drawn, as in later Vedantic philosophy, between Maya that envelops Ishvara and the Avidya that envelops Jiva: for both, the generic word Maya is used, and in the passage under consideration, it means only "power", almost in the same sense which Kuno Fisher gives to the "attributes" of Spinoza. Then again, in the Shvetashvatara, Maya is once more identified with Prakriti [Sh. 4.10], a usage which prevailed very much later, as may be seen from the way in which even the author of Kusumanjali had no objection in in identifying the two even for his theistic purpose. The Shvetashvatara also contains passages which describe the Godhead as spreading his meshes and making them manifold that he catches all the beings of the universe in them, and rules over them [Sh. 3.1, Sh. 5.3]. Here we have the conception of a net of meshes inside which all beings are entangled. Then again, a famous passage from Brihadaranyaka, which we have already considered, which speaks of "as if there were duality", implying thereby that there really is no duality, signifies the identification of Maya with a semblance, as-it-were, an appearance [Br. 2.4.14]. Finally, in that celebrated conversation between Shetaketu and Aruni which we have also had the occasion to consider, we are told that everything besides the Atman is merely a word, a mode and a name [Ch. 6.1.4]. We thus see from an examination of various passages in the Upanishads that even though the word Maya may not have been used for many times in the Upanishads, still the conception that underlies Maya is already present there and even though we do not find there the full-fledged doctrine of illusion in its philosophical aspects as in Gaudapada and later writers, still we do find in the Upanishads all the material that may have easily led Shankara to elaborate a theory of Maya out of it. When we consider that we have the conceptions of a veil, of blind-foldness, of a knot, of ignorance, of not-being, of darkness, of death, of unreality and uncertainty, of untruth, of crookedness, and falsehood and illusion, of the power of God, of this power as identical with nature, of meshes, of semblance, an as-it-were an appearance, and finally, of a word, a mode and a name, let no man stand up and say that we do not find the traces of the doctrine of Maya in the Upanishads!
Also, must read is this article, titled "Bhagavad Gita and Upanishads" (posted by Shri Ram Chandranji).