  The discovery of an exomoon would provide deep insights into planet formation
and the habitability of planetary systems, with transiting examples being
particularly sought after. Of the hundreds of Kepler planets now discovered,
the seven-planet system Kepler-90 is unusual for exhibiting an unidentified
transit-like signal in close proximity to one of the transits of the
long-period gas-giant Kepler-90g, as noted by Cabrera et. al. (2014). As part
of the 'Hunt for Exomoons with Kepler' (HEK) project, we investigate this
possible exomoon signal and find it passes all conventional photometric,
dynamical and centroid diagnostic tests. However, pixel-level light curves
indicate that the moon-like signal occurs on nearly all of the target's pixels,
which we confirm using a novel way of examining pixel-level data which we dub
the 'transit centroid'. This test reveals that the possible exomoon to
Kepler-90g is likely a false positive, perhaps due to a cosmic ray induced
Sudden Pixel Sensitivity Dropout (SPSD). This work highlights the extreme care
required for seeking non-periodic low-amplitude transit signals, such as
exomoons.
