Tears started to her eyes; she uttered a moan of bitterest disappointment, and slipped the envelope she was carrying into the letter-box.
The mechanic had seen her go in, and he waited outside, a few yards away.

Either she would soon reappear, or her not doing so would show that she had obtained admittance somewhere.

In the latter case, this workman of much curiosity and leisure had only to lurk about the staircase until she came forth again.

But this trial of patience was spared him.

He found that he had simply to follow the lady back to Herne Hill.