I quickly researched the law regarding the "Słowiński Park Narodowy" and it looks like the map is correct. 
In http://isap.sejm.gov.pl/isap.nsf/DocDetails.xsp?id=WDU20040430390 this act enacting the "Słowiński Park Narodowy" it says that a part of the park is plot number 35/2 (exact wording "działka  nr  35/2  (według  arkusza  nr  2  ze  stycznia 1977  r.  mapy  ewidencyjnej  obrębu  Rowy,  gmina Ustka) — o powierzchni 0,15 ha;") and this is more or less this area (as visible in a service provided by the local goverment https://gminaustka.e-mapa.net/?object=sys.dzialka.221210_2.0020.35/2 )
Probably some offices or other facilities of National Park Administration are located here.