The pilot and observer were to fly the sheriff's office helicopter from Summerville to Charleston (15 miles to the southeast) following maintenance work. When the aircraft did not arrive as scheduled, a search was initiated. The helicopter was found 4 days later in a forested swamp, about .6 mile northwest of the airport. The wreckage was found upright, destroyed by fire. There was evidence of impact with a large tree at about 70 feet AGL. An inspection of the wreckage revealed no evidence of a pre-existing mechanical failure or malfunction. The pilot had about 145 hours of helicopter flight time, but was not instrument rated, nor was the aircraft certificated for IFR. Numerous local residents reported heavy fog conditions at the time of the accident; one person who reportedly heard the crash estimated the visibility to be about 50-100 yards in fog. Darkness also prevailed at the reported time of the accident. The Safety Board believes that the pilot inadvertently entered IMC conditions and was attempting to navigate back to the departure airport when the aircraft impacted the trees.