Aims:We investigate the spatial distribution of magnetic polarities in the penumbra of a spot observed very close to disk center. Methods: High angular and temporal resolution magnetograms taken with the Narrowband Filter Imager aboard Hinode are used in this study. They provide continuous and stable measurements in the photospheric Fe I 630.25 line for long periods of time. Results: Our observations show small-scale, elongated, bipolar magnetic structures that appear in the mid penumbra and move radially outward. They occur in between the more vertical fields of the penumbra, and can be associated with the horizontal fields that harbor the Evershed flow. Many of them cross the outer penumbral boundary, becoming moving magnetic features in the sunspot moat. We determine the properties of these structures, including their sizes, proper motions, footpoint separation, and lifetimes. Conclusions: The bipolar patches can be interpreted as being produced by sea-serpent field lines that originate in the mid penumbra and eventually leave the spot in the form moving magnetic features. The existence of such field lines has been inferred from Stokes inversions of spectropolarimetric measurements at lower angular resolution, but this is the first time they are imaged directly. Our observations add another piece of evidence in favor of the uncombed structure of penumbral magnetic fields. A movie is only available in electronic form at http://www.aanda.org