Given that it is established early that the Quen Diviners have an early model of Focus that can only read data up to the end of the Clawback and lacking the ability to read data from afterwards particularly in regards to ZD and the Plague, but the Quen in some fashion was aware of the final retreat of Ted,
Given that the building of Theabes happened in 2064-2066 long after the time when the Focuses possessed by Diviners were able to access, and such information would have been available only to those who Ted deemed it necessary to be aware and being behind a series of security measures to protect him, that would if such information was found it would create some of a plot hole if the data was found by a Focus from the Clawback era
Possibly considering that within the Quen there is either forbidden or lost knowledge being restricted by the Board of Overseers, in addition to the knowledge that the public and other Diviners have been allowed access to, maybe they could have found at least one ZD era Focus after the establishment of the Ancestors as a pantheon of the most important Old Ones, with such information particularly if it was a Focus from a Cradle or ZD facility that lead them to find information about the Old Ones that would conflict with the established view would lead them to hide the truth from the public,
With the possibility that such a ZD era Focus could be within the possession of the Board of Overseers and with Lost Knowledge being eventually passed down to them, they would have eventually learned more information about various topics then anyone else within the Quen, and given that the total number of Diviners is a mystery even to themselves, any “new discoveries” could be attributed to a made up Diviner to hide the truth, with one such discovery being the location of Theabes,
Also given that the Quen have been discovering knowledge about the past for at least 13 generations by the time of HFW, maybe among that Data found would be knowledge that could assist in helping to combat NEMESIS or other aspects of ZD