Sometime after the destruction of GAIA Prime, Sylens made his way into the facility, eventually setting up a workshop in one of the exterior rooms. While he attempted numerous times to venture further, the security of the facility rendered his efforts fruitless. Based on the datapoints inside the workshop, Sylens likely took refuge here after being betrayed by HADES.
Aloy passes through the workshop when she visits the ruins to retrieve the Master Override. Sylens is waiting for her on the way out and confesses his degree of involvement with the Eclipse. He also provides her with a lance for inserting the Master Override into HADES. At the end of their discussion, Sylens and Aloy part ways, seemingly for the final time.