  Heavy neutral leptons (HNLs) are hypothetical particles, motivated in the
first place by their ability to explain neutrino oscillations. Experimental
searches for HNLs are typically conducted under the assumption of a single HNL
mixing with a single neutrino flavor. However, the resulting exclusion limits
may not directly constrain the corresponding mixing angles in realistic HNL
models -- those which can explain neutrino oscillations. The reinterpretation
of the results of these experimental searches turns out to be a non-trivial
task, that requires significant knowledge of the details of the experiment. In
this work, we perform a reinterpretation of the latest ATLAS search for HNLs
decaying promptly to a tri-lepton final state. We show that in a realistic
model with two HNLs, the actual limits can vary by several orders of magnitude
depending on the free parameters of the model. Marginalizing over the unknown
model parameters leads to an exclusion limit on the total mixing angle which
can be up to 3 orders of magnitude weaker than the limits reported in
arXiv:1905.09787. This demonstrates that the reinterpretation of results from
experimental searches is a necessary step to obtain meaningful limits on
realistic models. We detail a few steps that can be taken by experimental
collaborations in order to simplify the reuse of their results.
