Agatha Christie - Sleeping murder

Agatha Christie 1890-1976
Sleeping murder ( a Miss Marple Mystery ) 1976
Curtain: Poirot's Last Case

it is generally believed that Christie wrote Curtain (Hercule Poirot's last mystery, which concludes the sleuth's career and life) and Sleeping Murder duringWorld War II to be published after her death, and that Sleeping Murder was most probably written sometime during the Blitz, which took place between September 1940 and May 1941. But the Agatha Christie/Edmund Cork and Harold Ober literary correspondence files, currently held at Exeter University in Devon, indicate Sleeping Murder was written early in 1940.

let sleeping dogs lie=1.Prov. Do not instigate trouble.; Leave something alone if it might cause trouble.
2.to not talk about things which have caused problems in the past, or to not try to change a situation because you might cause problems.