  Ambiguous measurements do not reveal complete information about the system
under test. Their quantum-mechanical counterparts are semi-weak (or in the
limit, weak-) measurements and here we discuss their role in tests of the
Leggett-Garg inequalities. We show that, whilst ambiguous measurements allow
one to forgo the usual non-invasive measureability assumption, to derive an LGI
that may be violated, we are forced to introduce another assumption that
equates the invasive influence of ambiguous and unambiguous detectors. Based on
this assumption, we derive signalling conditions that should be fulfilled for
the plausibility of the Leggett-Garg test. We then propose an experiment on a
three-level system with a direct quantum-optics realisation that satisfies all
signalling constraints and violates a Leggett-Garg inequality.
