ON 12/3/85 AT APRX 0830, THE ACFT WAS FND LYING INVERTED IN A CANAL ADJACENT TO THE ARPT WHERE IT HAD BEEN ON 24 HR STANDBY FOR MEDICAL FLTS. TWELVE DAYS LATER, THE PLT'S BODY WAS ALSO FND IN THE CANAL. AN INVESTIGATION REVEALED THE ACFT HAD BEEN TAKEN SOMETIME DURING A PERIOD WHEN THE TOWER WAS CLOSED BETWEEN 2200 PST ON 12/2/85 & 0600 PST THE NEXT MORNING, WITHOUT THE KNOWLEDGE OR CONSENT OF THE OPERATOR. MAIN GEAR TIRE MARKS WERE FND IN MUD WHERE THE ACFT HAD CONTD OFF THE DEPARTURE END OF THE RWY. BEYOND THE END OF THE RWY, THE ACFT HIT A FENCE & A 5 FT DIRT BANK, THEN HIT THE GND AFTER BEING AIRBORNE ABOUT 200 FT & CAME TO REST IN THE CANAL. SHERIFF'S PSNL CONTACTED FRIENDS OF THE PLT, WHO HAD SPENT THE EVENING WITH HIM. BASE ON INFORMATION THAT THEY PROVIDED CONCERNING THE CONSUMPTION OF ALCOHOLIC BEVERAGES, THE SHERIFF'S PSNL STATED THAT THE PLT WOULD HAVE BEEN LEGALLY INTOXICATED. THE OVERRUN WAS PRESUMED TO HAVE OCCURRED DURING EITHER A LANDING OR A TAKEOFF ON 12/3/85.