  A small fraction of the halo field is made up of stars that share the light
element (Z<=13) anomalies characteristic of second generation globular cluster
(GC) stars. The ejected stars shed light on the formation of the Galactic halo
by tracing the dynamical history of the clusters, which are believed to have
once been more massive. Some of these ejected stars are expected to show strong
Al enhancement at the expense of shortage of Mg, but until now no such star has
been found. We search for outliers in the Mg and Al abundances of the few
hundreds of halo field stars observed in the first eighteen months of the
Gaia-ESO public spectroscopic survey. One halo star at the base of the red
giant branch, here referred to as 22593757-4648029 is found to have
[Mg/Fe]=-0.36+-0.04 and [Al/Fe]=0.99+-0.08, which is compatible with the most
extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029
to GCs of similar metallicity and find it unlikely that this star has been
tidally stripped with low ejection velocity from any of the clusters. However,
both chemical and kinematic arguments render it plausible that the star has
been ejected at high velocity from the anomalous GC omega Centauri within the
last few billion years. We cannot rule out other progenitor GCs, because some
may have disrupted fully, and the abundance and orbital data are inadequate for
many of those that are still intact.
