the UPS, that way the whole system is operating off-grid with the UPS filtering the ragged power from the generator.If the UPS needs to be unplugged to force use of its internal inverter, then wouldn't there therefore be no filtering of ragged generator power?
The point being that this system is likely relying on grid fluctuations to provide time matching, since grid conditions are sampled and recorded at various points in any given region for quality control purposes.
Using that information as the reference is probably how they are able to fingerprint the time and location by the fluctuations in the audio noise.
Any off-grid power source, such as a stand alone generator or a UPS running from its battery would not have that telltale hum as long as it was isolated from the grid and well away from grid connected equipment.
The UPS only needs to be disconnected from the grid, if it is getting power from a portable generator it won't have the same noise mapping as grid power.
Ultimately this means the government has figured out how to get information from the power companies, and now your power company is in league with the forensics teams.