The extra chime

An original fiction passage set in the nineteenth century.

Mr Dorrin stood proudly beside the repaired hall clock while its owner listened. He smiled as it sounded twelve steady strokes, each one clear and full. When silence returned, he reached for the winding key. A thirteenth note rang from within the case, and his satisfied smile faltered.

Identify the phrase which reveals Mr Dorrin's altered mood after the final chime.