  Nebular HeII$\lambda$4686\AA~line emission is useful to unveil active
galactic nuclei (AGN) residing in actively star-forming (SF) galaxies,
typically missed by the standard BPT classification. Here we adopt the HeII
diagnostic to identify hidden AGN in the Local Universe using for the first
time spatially-resolved data from the Data Release 15 of the Mapping Nearby
Galaxies at APO survey (MaNGA DR15). By combining results from HeII and BPT
diagnostics, we overall select 459 AGN host candidates ($\sim$10% in MaNGA
DR15), out of which 27 are identified as AGN by the HeII diagram only. The
HeII-only AGN population is hosted by massive (M$_*\gtrsim10^{10}$ M$_{\odot}$)
SF Main Sequence galaxies, and on average less luminous than the BPT-selected
AGN. Given the HeII line faintness, we revisit our census accounting for
incompleteness effects due to the HeII sensitivity limit of MaNGA. We thus
obtain an overall increased fraction (11%) of AGN in MaNGA compared to the
BPT-only census (9%), which further increases to 14% for galaxies more massive
than $10^{10}$ M$_{\odot}$; interestingly, on the SF Main Sequence the increase
is by about a factor of 2. A substantial number of AGN in SF galaxies points to
significant, coeval star formation and black hole accretion, consistently with
results from hydrodynamical simulations and with important implications on
quenching scenarios. In view of exploring unprecedented high redshifts with
JWST and new ground-based facilities, revisiting the standard BPT
classification through novel emission-line diagnostics is fundamental to
discover AGN in highly SF environments.
