  Low-redshift strong-lensing galaxies can provide robust measurements of the
stellar mass-to-light ratios in early-type galaxies (ETG), and hence constrain
variations in the stellar initial mass function (IMF). At present, only a few
such systems are known. Here, we report the first results from a blind search
for gravitationally-lensed emission line sources behind 52 massive $z$ $<$ 0.07
ETGs with MUSE integral field spectroscopy. For 16 galaxies, new observations
were acquired, whilst the other 36 were analysed from archival data. This
project has previously yielded one confirmed galaxy-scale strong lens
(J0403-0239) which we report in an earlier paper. J0403-0239 has since received
follow-up observations, presented here, which indicate support for our earlier
IMF results. Three cluster-scale, and hence dark-matter-dominated, lensing
systems were also discovered (central galaxies of A4059, A2052 and AS555). For
nine further galaxies, we detect a singly-imaged but closely-projected source
within 6 arcsec (including one candidate with sources at three different
redshifts); such cases can be exploited to derive upper limits on the IMF
mass-excess factor, $\alpha$. Combining the new lens and new upper limits, with
the previously-discovered systems, we infer an average $\langle \alpha \rangle$
= 1.06 $\pm$ 0.08 (marginalised over the intrinsic scatter), which is
inconsistent with a Salpeter-like IMF ($\alpha$ = 1.55) at the 6$\sigma$ level.
We test the detection threshold in these short-exposure MUSE observations with
the injection and recovery of simulated sources, and predict that one in
twenty-five observations is expected to yield a new strong-lens system. Our
observational results are consistent with this expected yield.
