  We present evidence for globular cluster stellar debris in a dwarf galaxy
system (Sagittarius: Sgr) based on an analysis of high-resolution
\textit{H}-band spectra from the Apache Point Observatory Galactic Evolution
Experiment (APOGEE) survey. We add [N/Fe], [Ti/Fe], and [Ni/Fe] abundance
ratios to the existing sample of potential members of M~54; this is the first
time that [N/Fe] abundances are derived for a large number of stars in M~54.
Our study reveals the existence of a significant population of nitrogen- (with
a large spread, $\gtrsim1$ dex) and aluminum-enriched stars with moderate Mg
depletion in the core of the M~54$+$Sagittarius system, which shares the light
element anomalies characteristic of second-generation globular cluster stars
(GCs), thus tracing the typical phenomenon of multiple stellar populations seen
in other Galactic GCs at similar metallicity, confirming earlier results based
on the Na-O anti-correlation. We further show that most of the stars in M~54
exhibit different chemical - patterns evidently not present in Sgr field stars.
Furthermore, we report the serendipitous discovery of a nitrogen-enhanced
extra-tidal star with GC second-generation-like chemical patterns for which
both chemical and kinematic evidence is commensurate with the hypothesis that
the star has been ejected from M~54. Our findings support the existence of
chemical anomalies associated with likely tidally shredded GCs in dwarf
galaxies in the Local Group and motivate future searches for such bonafide
stars along other known Milky Way streams.
