He was induced to don a false mustache until the detective had gone.

It was a long, heavy black mustache with curling tips, and in this disguise he stood aloof from his companions when the detective entered.
The detective was the cross-eyed man, himself now disguised as Sherlock Holmes, with a fore-and-aft cloth cap and drooping blond mustache.
He smoked a pipe as he examined those present.

Merton was unable to overlook this scene, as he had been directed to stand with his back to the detective.

Later it was shown that he observed in a mirror the Mexican whom he had punished creeping forward to inform the detective of his man's whereabouts.

The coward's treachery cost him dearly.