Code D was used by UNIT in reference to the Daleks. UNIT had traced a potential Code D that crash landed in 1962, but lost track of it in Utah. Encountering a crashed Dalek in the 2010s, Kate Stewart recorded it as Code D. (AUDIO: The Dalek Transaction)
Alternatively, the Thirteenth Doctor classed the appearance of a reconnaissance scout Dalek on 1 January 2019 as a Code Zero emergency, which she attempted to report to Kate, only to be directed to Polly at the UK Security Helpline, who informed her that UNIT operations had been suspended. (TV: Resolution)