Abstract: I will discuss new opportunities to search for weakly-coupled, long-lived particles (LLP) produced in rare meson and lepton decays. We derive new limits and sensitivity to the visible decays of these LLPs at spallation sources and neutrino experiments. As a prototypical example, we focus on axion-like particles coupled to leptons. Time permitting, I will also link some of these signatures with ultra-high energy events at neutrino telescopes. Together, these approaches map a broad and rich landscape for discovering a wide range of weakly-coupled particles.