  Transition disks offer the extraordinary opportunity to look for newly born
planets and investigate the early stages of planet formation. In this context
we observed the Herbig A5 star MWC 758 with the L band vector vortex
coronagraph installed in the near-infrared camera and spectrograph NIRC2 at the
Keck II telescope, with the aim of unveiling the nature of the spiral structure
by constraining the presence of planetary companions in the system. Our
high-contrast imaging observations show a bright (delta L=7.0+/-0.3 mag)
point-like emission, south of MWC 758 at a deprojected separation of about 20
au (r=0.111+/- 0. 004 arcsec) from the central star. We also recover the two
spiral arms (south-east and north-west), already imaged by previous studies in
polarized light, and discover a third one to the south-west of the star. No
additional companions were detected in the system down to 5 Jupiter masses
beyond 0.6 arcsec from the star. We propose that the bright L band emission
could be caused by the presence of an embedded and accreting protoplanet,
although the possibility of it being an asymmetric disk feature cannot be
excluded. The spiral structure is probably not related to the protoplanet
candidate, unless on an inclined and eccentric orbit, and it could be due to
one (or more) yet undetected planetary companions at the edge of or outside the
spiral pattern. Future observations and additional simulations will be needed
to shed light on the true nature of the point-like source and its link with the
spiral arms.
