they are all under the 27 (maybe even 27L only)
Correct.
Two mobile noise monitors have been deployed under the 27L approach, at Mogden Sewage Treatment Works in Isleworth and the Royal Mid-Surrey Golf Club at Old Deer Park, Richmond, supplemented by the existing fixed monitor site adjacent to Hounslow Heath (normally used to measure 09R departure noise levels).
Heathrow have said that also hope to site an additional mobile monitor farther away from the airport in order to measure noise earlier on the 27L approach, but no location has yet been announced.