Left ear cup interior has an 'indent. What is it for?

[Stoobloo]

I've just started using my WH-1000XM4 headphones, and have noticed that the inside of the left earcup does not 'look' the same as the right earcup.

Please see attached showing 'indented' area.

[ArthurWellesley]

I don't know for sure but my guess would be that it's a sensor for the automatic pause feature, allowing the headphones to detect when you have taken them off your ears.