I don't really know, but it appears consistent with a noise source of constant strength that's on almost continuously, apart from a few upward spikes of SNRM which are consistent with a modem re-connecting or rebooting. Occasionally it's been off for half an hour or so. The SNR per tone graph doesn't show any new interference on any particular tones, just a general reduction of SNR across the whole range.
I might have spotted a previously unseen BTHub3 from scanning the wifi at the time, but this isn't terribly conclusive, and I don't seem to have made any note of the exact wireless SSID.
It started on 22 July 2016, 10 January 2017 was the most recent time I've seen the SNRM spike upwards.