THE ACFT WAS OBSERVED BY WITNESSES TO DESCEND BELOW ITS NORMAL DESCENT PATH AND TO HORIZONTIALLY STRAY OFF COURSE TO THEPOINT THAT OFFICE BLDG OCCUPANTS AND PERSONS ON THE GROUND WERE CONCERNED BY THE APPEARANCE AND NOISE OF THE ACFT DURINGA VOR/DME APPROACH TO WASHINGTON NAT'L ARPT. THE FDR AND RADAR PLOTS CONFIRM THE POSITION OF THE ACFT. ONE OF THE BUILDINGS 2.5 MILES FROM THE ARPT IS ABOUT 396 FT MSL. THE VOR/DME APPROACH HAS AN ALT RESTRICTION OF 900 FT MSL AT 3 NMAND A MINIMUM DESCENT ALT OF 720 FT AT 2.5 NM. THE PLT WAS INTERVIEWED AND HE STATED THAT HE THOUGHT EVERYTHING WAS NORMAL AND THAT HE WAS AT THE ALT RESTRICTION AT THE 3 MILE DME. RADAR SHOWS HIS ALT AS ABOUT 517 FT AT 3 MILE DME AND ABOUT 400 FT AT 2.5 MILE DME. RADAR AND WITNESSES AGREE ON A TURN AND CLIMB AT THIS POINT TOWARD THE RIVER AND THE NORMAL APPROACH PATH.