THE PLT, WHO WAS A CONTRACTOR, WAS SCHEDULED TO ATTEND A HEAVY EQUIP'T AUCTION IN SACRAMENTO TO BEGIN A 1000. WITNESSES HEARD NORMAL ENG SOUNDS AS THE ACFT DEPARTED RWY 19 & TURNED TO A LEFT DOWNWIND DEPARTURE TURN. THE ACFT WAS OBSERVED PROCEEDING ON A NE HEADING, PARALLELING HWY 152 AT ABOUT 500 FT AGL GOING STRAIGHT TOWARD WHERE IT ENDED UP ON A HILLSIDE. ANOTHER WITNESS OBSERVED THE ACFT FLYING BENEATH THE CLOUDS & THEN ENTER THE CLOUDS IN A CLIMB NEAR THE BASE OF THE HILL. ARPT BASED PLTS DESCRIBED THE DEPARTURE AS ONE OF TWO 'BOOTLEG' IFR CLIMB PROCEDURES COMMONLY USED BY MANY LOCAL PLTS DURING LOW STRATUS CONDITIONS. THE PLT HAD BEEN OBSERVED USING THE 'HIGHWAY 152 CLIMB' MANY TIMES DURING THE PRECEEDING YEARS. THERE IS NO RECORD OF THE PLT ATTEMPTING TO OBTAIN A 'POP-UP' IFR TO VFR CLEARANCE. THE PLT FLEW AN AVERAGE OF 2 HRS EACH WORKING DAY.