There are some (not all) 50Hz mains sidebands that couples with hrec; a good witness (also used for a 50Hz reduction through a feedforward) is the signal named ENV_CEB_ELECTRIC. This signal is the voltage drop between the electronic WI tower suspension crate and the tower's vacuum vessel (using a Stanford SR-560 10x differential amplifier).
These sidebands are usually generated by heavy loads (e.g. heaters) switched ON and OFF with a cycle of about 1s
Trying to reduce the sidebands in large families we can see that one is at +/- 1.25Hz (and its +/- 2.5Hz harmonic, period is 0.8S), the second is at +/- 1.7Hz (with its harmonics at +/- 3.4Hz and +/- 5.1Hz, period is 0.6S)
Using data of the last HVAC shutdown test (19 April 2019) we can see that none of the sidebands disappear in ENV_CEB_ELECTRIC; we can therefore exclude all HVAC systems (except the INJ that has been left on) as a possible source.
Another interesting hint is that this signal has no coherence at sidebands frequencies with the magnetometer located in the Mode Cleaner Building (ENV_MCB_MAG), this excludes a possible earth return current from the MCB HVAC machine through the MCB vacuum pipe towards the central building; instead there is coeherence with other sidebands, all very close to the 50Hz mains (e.g. +/- 0.53Hz), we should repeat the test made in this entry with a look at all visible sidebands.