  Substructure in globular cluster (GC) populations around large galaxies is
expected in galaxy formation scenarios that involve accretion or merger events,
and it has been searched for using direct associations between GCs and
structure in the diffuse galaxy light, or with GC kinematics. Here, we present
a search for candidate substructures in the GC population around the Virgo cD
galaxy M87 through the analysis of the spatial distribution of the GC
colors.~The study is based on a sample of $\sim\!1800$ bright GCs with
high-quality $u,g,r,i,z,K_s$ photometry, selected to ensure a low contamination
by foreground stars or background galaxies.~The spectral energy distributions
of the GCs are associated with formal estimates of age and metallicity, which
are representative of its position in a 4-D color-space relative to standard
single stellar population models.~Dividing the sample into broad bins based on
the relative formal ages, we observe inhomogeneities which reveal signatures of
GC substructures.~The most significant of these is a spatial overdensity of GCs
with relatively young age labels, of diameter $\sim\!0.1$\,deg
($\sim\!30\,$kpc), located to the south of M87.~The significance of this
detection is larger than about 5$\sigma$ after accounting for estimates of
random and systematic errors.~Surprisingly, no large Virgo galaxy is present in
this area, that could potentially host these GCs.~But candidate substructures
in the M87 halo with equally elusive hosts have been described based on
kinematic studies in the past.~The number of GC spectra available around M87 is
currently insufficient to clarify the nature of the new candidate substructure.
