This came up in chat a little while ago and I realized I’d never actually posted about it, so…
Did you notice how Scotland Yard was missing from Series 3? That location only showed up in flashbacks and a cameo appearance by a single wall in Sherlock’s mind palace (seen above).
It may have been because the writers didn’t really need to set any scenes there to tell the story this time, and why go to the trouble of recreating the set if you don’t actually need it.
Coincidentally or not, though, there’s something else going on with Scotland Yard.
Sherlock came back from the dead in November of 2013, which means Series 3 ended sometime after Christmas 2014. If the writers pick up where they left off (and they’ve said they will), the next episode will probably be set in 2015. Which matches up with the year it’s supposed to air, so that wouldn’t be a surprise.
But the real-world Scotland Yard is moving to a new location in 2015. I’ve included some images of the new headquarters design above, and you can see more here.
I expect we may be seeing a new in-show design for Scotland Yard soon as well. (Soon by Sherlock standards, anyway.) Barring more flashbacks, it’s possible Sherlock’s brain-wall was a goodbye to the old set.