I was wondering if that might have been Photoshopped, but Alex had a closer look and noticed that not only is Cumberbatch's reflection visible in the glass, there are also a couple of other reflections further to the right -- one that looks very plausibly like Martin Freeman and another who could be a woman (producer?) we saw working with them at the May 21st North Gower shoot.
So we think someone on the crew snapped this when they were filming the scene in the Tube station. Just in case y'all hadn't figured that out already!
Haha I can just picture Ben & Martin begging Moffat & Gatiss to keep this scene in the episode for amusement purposes.
Moffat & Gatiss saying Nope no not ever going to happen.
Martin & Ben flipping them the bird and just accepting it.