  MUSE-based emission-line maps of the spiral galaxy NGC 4030 reveal the
existence of unresolved sources with forbidden line emission enhanced with
respect to those seen in its own HII regions. This study reports our efforts to
detect and isolate these objects and identify their nature. Candidates are
first detected as unresolved sources on an image of the second principal
component of the Hb, [OIII]5007, Ha, [NII]6584, [SII]6716, 6731 emission-line
data cube, where they stand out clearly against both the dominant HII region
population and the widespread diffuse emission. The intrinsic emission is then
extracted accounting for the highly inhomogeneous emission-line "background"
throughout the field of view. Collisional to recombination line ratios like
[SII]/Ha, [NII]/Ha, and [OI]/Ha tend to increase when the background emission
is corrected for. We find that many (but not all) sources detected with the
principal component analysis have properties compatible with supernova remnants
(SNRs). Applying a combined [SII]/Ha and [NII]/Ha classification criterion
leads to a list of 59 sources with SNR-like emission lines. Many of them
exhibit conspicuous spectral signatures of SNRs around 7300 Angs, and a
stacking analysis shows that these features are also present, except weaker, in
other cases. At nearly 30 Mpc, these are the most distant SNRs detected by
optical means to date. We further report the serendipitous discovery of a
luminous (M_V ~ -12.5), blue, and variable source, possibly associated with a
supernova impostor.
