  Experiments searching for Heavy Neutral Leptons (HNLs) typically interpret
their results within simplified models consisting of a single HNL coupled to a
single lepton flavor. However, any model which aims to describe neutrino
oscillations necessarily features more than one HNL, coupled to several
flavors. As we show in this work, the reinterpretation of the results of
experimental searches in terms of realistic models is a non-trivial task. We
perform a detailed reinterpretation of the latest ATLAS search for prompt HNLs
in W decays within a minimal low-scale seesaw with two HNLs. We show that the
exclusion limits obtained using the detailed reinterpretation can differ by
several orders of magnitude from the limits quoted for the simplified models.
Hence naively comparing the mixing angles from a realistic model to the
reported limits could lead to wrongly excluding entire regions of parameter
space! To overcome this issue without requiring experiments to report
constraints on all possible HNL models, we propose a simple framework that
allows one to easily and accurately reinterpret exclusion limits within
closely-related models. We outline a number of concrete steps that can be taken
by experiments to implement this method with minimal effort, and we discuss its
applicability to other models of feebly interacting particles.
