I am currently watching The Last Detective on Britbox. A character called Mod played by the late Sean Hughes quoted the last two lines of this poem. It is the eighth of Auden's Twelve Songs - April 1936.
At last the secret is out, as it always must come in the end, The delicious story is ripe to tell to tell to the intimate friend; Over the tea-cups and in the square the tongue has its desire; still waters run deep, my dear, there’s never smoke without fire.
Behind the corpse in the reservoir, behind the ghost on the links, Behind the lady who dances and the man who madly drinks, Under the look of fatigue the attack of migraine and the sigh There is always another story, there is more than meets the eye.
For the clear voice suddenly singing, high up in the convent wall, The scent of the elder bushes, the sporting prints in the hall, The croquet matches in summer, the handshake, the cough, the kiss, There is always a wicked secret, a private reason for this.