The origin of the neutrinos masses, baryon asymmetry in the universe, and the nature of dark matter remain fundamental open problems in HEP. The FCC-ee provides exciting opportunities to resolve these mysteries with the discovery of heavy neutral leptons (HNLs) via $e^+e^- \to Z \to \nu N$ by exploiting a huge sample ($8\times 10^{12}$) of Z bosons. The expected very small mixing between light and heavy neutrinos leads to tiny mixing angles, resulting in very long HNL lifetimes and a spectacular signal topology. Recent work based on a parametrised simulation of the IDEA detector will be described. The sensitivity region in the HNL parameter space will be mapped for prompt and long-lived signatures. A percent-level mass resolution can be achieved with inner-detector timing over large part of the HNL parameter space. Results of models with HNLs oscillations inside the detector will be also presented.