The lines above are part of a longer poem (or song?) that Agatha Christie wrote herself. Here's a bit more, but it might not be complete:

From Poems by Agatha Christie (London: William Collins and Sons, 1973), page 44:


And whatever I am seeing, there's no fearing and no fleeing . . .
But whatever I am seeing, it is not my heart's desire. . . .
Sheila, dark Sheila, with whom will you be roaming?
With whom will you be roaming when the summer day has flown?

A lad there is who loved me - but loves me now no longer,
A lad there is who left me (and oh! his love grows stronger!
But wherever I go roaming, you shall never find me homing,
For wherever I go roaming, I must wander all alone. ...