I enjoyed the recent series of Father Brown. I did notice something about two of the episodes.
Father Brown, series 4, episode 5, The Daughters of Autolycus.
See the short story The Duchess of Wiltshire’s Diamonds by Guy Boothby in his book A Prince of Swindlers or in The Rivals of Sherlock Holmes edited by Hugh Greene. It was adapted in the television series The Rivals of Sherlock Holmes and sometimes can be found on YouTube.
Father Brown, series 4, episode 6, The Rod of Asclepius.
See the novel Green For Danger (1944) by Christianna Brand and the film adaptation of that novel starring Alistair Sim (1946).
I won't give any further details because that will spoil seeing the episodes and reading the stories mentioned above.