There's no central register of unmetered power generation, the assumption used is that if on a windy day mysteriously we're buying 25GW from the grid across the country, while on a similar but calm day it was 30GW that suggests 5GW of electricity is not being purchased when it's windy, implying 5GW of unmetered electricity supply from wind.

And there are presumably other embedded systems making up some of that 5GW estimate. Farms just seem obvious because 1) I've seen farms with wind turbines 2) They're industrial scale electricity users, often occupying terrain very suitable for a wind turbine