When the second Doctor Who Series 11 trailer was released, Bradley Walsh was featured in a scene where he has some sort of implant attached to his right temple.
Behind-the-scenes footage from Access All Areas also show these implants on the right temples of Mandip Gill and Jodie Whittaker.
I can tell you that, from a few photos that were accidentally posted publicly on the still photographer’s website, it’s the same episode in which Mark Addy appears.
So which episode features the implants? The foremost candidates are Episodes 7, 8, 10 (the series finale), and the Christmas special.