The Bliss of Mrs. Blossom (1968)

Harriet Blossom, the lonely wife of a workaholic brassiere manufacturer, breaks her sewing machine and ends up in bed with the repairman, a mechanic from one of her husband's factories. The man, Ambrose, is supposed to leave during the night, but Harriet finds him the next day still hiding in the attic. Harriet lets her new lover stay in the attic for years, managing to keep it all secret from her husband and from the detectives investigating the sudden disappearance of a certain Mr. Ambrose Tuttle.