The House of the Schemers by Fred M. White

Ailsa stared almost stupidly at her companion. She had not expected anything like this. Never before had Archibald Colville asked for her at any time in the day, and for him to want her now roused all the girl's suspicions. At any rate there must have been something radically wrong for Grace to run such a terrible risk and fetch her out of No. 14 like this.

And only a little time before Ailsa had seen Archibald Colville standing in the street and gazing at the house next door as if he had been an entire stranger to the locality. Ailsa had asked Grace to keep an eye upon him, especially as Colville had deliberately gone out of the way to tell her that he was not coming home to-night.