I'm sadly in the same place, and I hope I'm wrong or that she is still with us and can be found.
MOO:
My speculation only: She's approaching 50, a time when a lot of people take stock of their lives. She has lived in Europe, Boston, San Francisco, and who knows where else. Even if she jumped all in on her second career as a teacher, maybe it wasn't what she had expected (a few posts upthread talk about the overwhelming demands on teachers today). I live in a northern inter-mountain state, and, although I love it here, there are days when it is downright dreary and I wonder why I'm not in Acapulco. She may have felt it confining after a while, which she would not want to admit to her husband and family because she loves them. Where I am, the days are markably shorter, and the amount of daylight can affect moods, even in someone who does not technically have Seasonal Affective Disorder.
I wince to write this, but since we are in the time of having awkward conversations, menopause could also be a factor. It disrupts hormonal balances and also drives home the point to women that the child-bearing years are behind us. (I'm NOT saying menopause is mental illness!) Then her home and work life is affected by renovation -- more disruption. I just see a lot of factors that could combine to cause distress and unrest, and a feeling that it was too late to make another change (even though it's not, and not all helpful changes have to be HUGE). All JMO and take all with a huge boulder of salt.
Until we learn otherwise, I'm holding out hope that she ran away with a dashing ferry captain, and they are setting up their new enterprise ferrying German tourists up and down the Danube River. Her DH will rebound with a glamorous SAR pro, and they will happily train search dogs for deployment around the world.