  The exploration of long-lived particles in the MeV-GeV region is a formidable
task but it may provide us a unique access to dark sectors. Fixed-target
facilities with sufficiently energetic and intense proton beams are an ideal
tool for this challenge. In this work we show that the production rate of
Axion-Like-Particles (ALPs) coupled pre-dominantly to photons receives a
significant contribution from daughter-photons of secondary $\pi^0$ and $\eta$
mesons created in the proton shower. We carefully compare the PYTHIA simulated
spectra of such secondaries to experimental literature, compute the ALP flux
from the Primakoff conversion of these photons, and finally revisit existing
limits on ALPs and update the prospects for a set of existing and future
searches. Our results show that taking this production mechanism into account
significantly enhances the sensitivity compared to previous studies based on
coherent ALP production in primary proton-nucleus interactions.
