We derive constraints on the mixing of heavy right-handed neutrinos with the SM fields in the most general Seesaw scenario where the heavy neutrinos are integrated out. Among the electroweak and flavour observables included in the global fit, $\mu\rightarrow e\gamma$ sets the present strongest bound on the additional neutrino mixing, while in the future it will be dominated by $\mu-e$ conversion in nuclei. Increasing its sensitivity in future experiments could probe Non-Unitarity in Lepton Flavour Violating processes. Nevertheless, in order to determine completely model-independent constraints, we provide a second set of bounds derived through a global fit that does not include LFV observables. These indirect constraints on the off-diagonal elements come from the diagonal bounds through the Schwarz inequality.