The Chink in the Armour by Marie Belloc Lowndes

There is something very bewildering and distressing in the sudden disappearance or even the absence of a human being to whose affectionate and constant presence one has become accustomed. And as the hours went by, and no letter or message arrived from Anna Wolsky, Sylvia became seriously troubled, and spent much of her time walking to and from the Pension Malfait.

Surely Anna could not have left Paris, still less France, without her luggage? All sorts of dreadful possibilities crowded on Sylvia's mind; Anna Wolsky might have met with an accident: she might now be lying unidentified in a Paris hospital.