  Energetic nuclear outbursts have been discovered in luminous and
ultraluminous infrared galaxies (U/LIRGs) at unexpectedly high rates. To
investigate this population of transients, we performed a search in mid-IR data
from the Wide-field Infrared Survey Explorer (WISE) satellite and its NEOWISE
survey to detect and characterise luminous and smoothly evolving transients in
a sample of 215 U/LIRGs. We report three new transients, all with $\Delta L >
10^{43}$ erg s$^{-1}$, in addition to two previously known cases. Their host
galaxies are all part of major galaxy mergers, and through radiative transfer
model fitting we find that all have a significant contribution from an active
galactic nucleus (AGN). We characterised the transients through measurements of
their luminosities and resulting energetics, all of which are between
10$^{50.9}$ erg and 10$^{52.2}$ erg. The IR emission of the five transients was
found to be consistent with re-radiation by the hot dust of emission at shorter
wavelengths, presumably originating from an accretion event, onto the
supermassive black hole. The corresponding transient rate of
(1.6-4.6)$\times$10$^{-3}$ / yr / galaxy is over an order of magnitude higher
than the rate of large amplitude flares shown by AGN in the optical. We suggest
that the observed transients are part of a dust-obscured population of tidal
disruption events (TDEs) that have remained out of the reach of optical surveys
due to the obscuring dust. In one case, this is supported by our radio
observations. We also discuss other plausible explanations. The observed rate
of events is significantly higher than optical TDE rates, which can be expected
in U/LIRG hosts undergoing a major galaxy merger with increased stellar
densities in the nuclear regions. Continued searches for such transients and
their multi-wavelength follow-up is required to constrain their rate and
nature.
