****SPOILERS as always****
With Hattie Stubbs missing, it seems likely a recent photograph of her would have been circulated in the papers and far and wide. Etienne de Sousa just had to catch a glimpse of one, and surely a lot of plot would have fallen apart.
The impetus for the shape of the disappearance of Hattie, as devised by the villains, was the fact that Etienne de Sousa had written to announce his imminent arrival. They knew that although it had been some years that Etienne had met Hattie, he would not have been deceived by an imposter– Poirot points out this motive in the final chapter. Hattie must disappear. Since the Tuckers are being nuisances as well, they devise murder and disappearance in a way that casts suspicion on De Sousa.
I just don’t see how they could have gotten round the “photograph in the paper” problem all too well…
‘Don’t bother about me,’ [Mrs Oliver] said to Poirot. ‘I’m just remembering if there’s anything I’ve forgotten.’
Sir George laughed heartily.
‘The fatal flaw, eh?’ he remarked.
‘That’s just it,’ said Mrs Oliver. ‘There always is one. Sometimes one doesn’t realize it until a book’s actually in print. And then it’s agony!’ Her face reflected this emotion. She sighed. ‘The curious thing is that most people never notice it. I say to myself, “But of course the cook would have been bound to notice that two cutlets hadn’t been eaten.” But nobody else thinks of it at all.’
–Dead Man’s Folly
Seven Storeys High 