Put yourself into their place for a moment.

They had seen my advertisement about Celie Harland in the Geneva paper.

Marthe Gobin, that busybody who was always watching her neighbours, was no doubt watched herself.
They see her leave the house, an unusual proceeding for her with her husband ill, as her own letter tells us.

Hippolyte follows her to the station, sees her take her ticket to Aix and mount into the train.

He must guess at once that she saw Celie Harland enter their house, that she is travelling to Aix with the information of her whereabouts.