The locket

Among the recovered personal effects of the senior Woolcotts was a locket with shadow portraits of what might have been Emma and Philip's grandparents.

It seems innocuous enough, but of course there is more to it than meets the eye. If you turn the glass just so, the portraits vanish and what remains is a sophisticated cypher.

When Emma brings her locket "to the jeweller for repairs," she is actually getting the updated codes from her mysterious emloyer.

rough concept sketch of Emma's locket