Hartmut Grote from the GEO 600 collaboration kindly replied to my inquiry about the status and let me know
"In GEO600 we recently found that there is no more unexplained noise in the region from 150 to 300 Hz, if we use a different readout method, which points to the fact that the unexplained noise in this region might be associated with the former readout method, and not be of any fundamental type (i.e. holographic).
However, this does not change much in the current discussion of wether GEO is limited by holographic noise or not, as Craig Hogan already agreed some time ago, that the low-frequency rise in the noise in GEO would not be holographic noise. Hogans latest prediction is a flat (in frequency spectrum) noise, and we have not yet made an experimental statement about this in GEO.
So in summary:
Mystery noise in GEO disappeared in the region 150-300Hz, but Hogan anyway was not suggesting any more that holographic noise would be limiting GEO at these frequencies since a while."
Thus, a big bunch of the "mystery noise" has found a non-mysterious explanation. It is not entirely clear yet how much is left to explain and whether there will be anything mysterious about what is left. GEO600 might then just about reach the required sensitivity to test the remainder of Hogan's prediction. The experimentalists plan to improve the sensitivity in the coming year and hope to eventually be able to settle the question.