In this talk, we present a novel methodology and preliminary findings in investigating decays occurring at Belle II specifically those involving undetectable products. Such processes encompass standard model decays with neutrinos and beyond standard model scenarios featuring unseen particles in the final state. We are especially focused on the lepton-flavor-violating 𝜏→ℓ𝛼 (ℓ= e, 𝜇) decay in a 1x1 topology (using 𝜏→𝜋𝜈 as a tag), where 𝛼 represents an invisible pseudo-Goldston boson, which could be a key piece in the understanding the hierarchy problem, including the mass structure of the charged leptons, quark masses, the problem of the CP strong phase, or even as a candidate for dark matter, among others possibilities. This comprehensive exploration sheds light on the versatility of the proposed techniques across different scenarios.