Sherlock’s co-creator Mark Gatiss confirms that Sherlock made a surprise appearance in a new BBC Christmas special. Starring Benedict Cumberbatch as the titular detective, Sherlock ran for four seasons on BBC from 2010–2017. The series was co-created by Gatiss and Steven Moffat, and starred Cumberbatch alongside Martin Freeman, Andrew Scott, Rupert Graves, and Une Stubbs.
https://screenrant.com/ghost-story-for-christmas-lot-249-sherlock-bbc-cameo-explained/