Context : Transition disks offer the extraordinary opportunity to look for newly born planets and investigate the early stages of planet formation .

Aims : 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 .

Methods : Our high-contrast imaging observations show a bright ( \Delta L ^ { \prime } = 7.0 \pm 0.3 mag ) point-like emission , south of MWC 758 at a deprojected separation of \sim 20 au ( r = 0 \aas@@fstack { \prime \prime } 111 \pm 0 \aas@@fstack { \prime \prime } 004 ) 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 \aas@@fstack { \prime \prime } 6 from the star .

Results : 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 can not 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 .

Conclusions :