Wikipaedia suggests that Ms Christie's disappearance related to her Husband's infidelity, which later on led to them divorcing (whether or not she had a breakdown or depression, which apparently her Mother suffered from, according to Wikipaedia, and scarpered off to Yorkshire). This produced a scare, with some even suggesting that her husband had done away with her!
There is a book available 2nd-hand on Amazon relating to this, link as follows:-
Agatha Christie and the Eleven Missing Days I don't know if this would assist you or not.
Good luck!