  Many Active Galactic Nuclei (AGN) surveys rely on optical emission line
signatures for robust source classification. There are, however, examples of
luminous AGN candidates lacking such signatures, including those from the
narrow line region, which are expected to be less susceptible to classical
nuclear (torus) obscuration. Here, we seek to formalise this subpopulation of
AGN with a prototypical candidate, SDSS J075139.06+402810.9. This shows IR
colours typical for AGN, an optical spectrum dominated by an early type galaxy
continuum,an [OIII] 5007\r{A} limiting flux about two dex below Type 2 quasars
at similar IR power, and a k-corrected 12 micron quasar-like luminosity of
$\sim$ 10$^{45}$ erg s$^{-1}$. These characteristics are not consistent with
jet and host galaxy dilution. A potential scenario to explain this AGN
quiescence in the optical is a sky-covering "cocoon" of obscuring material,
such that the AGN ionising radiation is unable to escape and excite gas on kpc
scales. Alternatively, we may be witnessing the short phase between recent
triggering of obscured AGN activity and the subsequent narrow line excitation.
This prototype could define the base properties of an emerging candidate AGN
subtype - an intriguing transitional phase in AGN and galaxy evolution.
