  We investigate signatures of population III (PopIII) stars in the
metal-enriched environment of GRBs originating from population II-I (PopII/I)
stars by using abundance ratios derived from numerical simulations that follow
stellar evolution and chemical enrichment. We find that at $z>10$ more than
$10%$ of PopII/I GRBs explode in a medium previously enriched by PopIII stars
(we refer to them as GRBII$\rightarrow$III). Although the formation of
GRBII$\rightarrow$III is more frequent than that of pristine PopIII GRBs
(GRBIIIs), we find that the expected GRBII$\rightarrow$III observed rate is
comparable to that of GRBIIIs, due to the usually larger luminosities of these
latter. GRBII$\rightarrow$III events take place preferentially in small
proto-galaxies with stellar masses $\rm M_\star \sim 10^{4.5} - 10^7\,\rm
M_\odot$, star formation rates $\rm SFR \sim 10^{-3}-10^{-1}\,\rm M_\odot/yr$
and metallicities $Z \sim 10^{-4}-10^{-2}\,\rm Z_\odot$. On the other hand,
galaxies with $Z < 10^{-2.8}\,\rm Z_\odot$ are dominated by metal enrichment
from PopIII stars and should preferentially host GRBII$\rightarrow$III. Hence,
measured GRB metal content below this limit could represent a strong evidence
of enrichment by pristine stellar populations. We discuss how to discriminate
PopIII metal enrichment on the basis of various abundance ratios observable in
the spectra of GRBs' afterglows. By employing such analysis, we conclude that
the currently known candidates at redshift $z\simeq 6$ -- i.e. GRB 050904
\cite[][]{2006Natur.440..184K} and GRB 130606A \cite[][]{2013arXiv1312.5631C}
-- are likely not originated in environments pre-enriched by PopIII stars.
