Context : We analyzed spectropolarimetric data recorded with Hinode/SP in quiet-Sun regions located at the disk center . We found single-lobed Stokes V profiles showing highly blue- and red-shifted signals . Oftentimes both types of events appear to be related to each other . Aims : We aim to set constraints on the nature and physical causes of these highly Doppler-shifted signals , as well as to study their spatial distribution , spectropolarimetric properties , size , and rate of occurrence . Also , we plan to retrieve the variation of the physical parameters with optical depth through the photosphere . Methods : We have examined the spatial and polarimetric properties of these events using a variety of data from the Hinode spacecraft . We have also inferred the atmospheric stratification of the physical parameters by means of the inversion of the observed Stokes profiles employing the Stokes Inversion based on Response functions ( SIR ) code . Finally , we analyzed their evolution using a time series from the same instrument . Results : Blue-shifted events tend to appear over bright regions at the edge of granules , while red-shifted events are seen predominantly over dark regions on intergranular lanes . Large linear polarization signals can be seen in the region that connects them . The magnetic structure inferred from the time series revealed that the structure corresponds to a \Omega -loop , with one footpoint always over the edge of a granule and the other inside an intergranular lane . The physical parameters obtained from the inversions of the observed Stokes profiles in both events show an increase with respect to the Harvard-Smithonian reference atmosphere in the temperature at \log \tau _ { 500 } \in ( -1 , -3 ) and a strong magnetic field , B \geq 1 kG , at the bottom of the atmosphere that quickly decreases upward until vanishing at \log \tau _ { 500 } \approx - 2 . In the blue-shifted events , the line of sight velocities change from upflows at the bottom to downflows at the top of the atmosphere . Red-shifted events display the opposite velocity stratification . The change of sign in line of sight velocity happens at the same optical depth in which the magnetic field becomes zero . Conclusions : The physical mechanism that best explains the inferred magnetic field configuration and flow motions is a siphon flow along an arched magnetic flux tube . Further investigation is required however , as the expected features of a siphon flow can not be unequivocally identified .