We show how a light axion-like particle (ALP) leaves tell-tale
signatures in processes that involve standard model (SM) fields only
(i.e., SM processes). These include the violation of the
Gell-Mann--Okubo mass relation, modification of form factors,
alteration of differential rates for various SM transitions, give
rise to novel sum rules etc. Therefore, in the presence of an ALP,
extractions of masses, mixing angles, and form factors in a
data-driven way provide important (indirect) bounds on ALP physics.
These bounds remain valid in the limits where new physics effects
conspire to weaken the bounds from direct searches. We provide a
proof of concept example by analysing
\texorpdfstring{\(K^+_{\ell_3}\)}{Kl3} decays. We also derive sum
rules which give hints towards the nature of the ALP physics in the
UV.