This incident occurred when there was no intent to fly. On 3-8-82, a Boeing 707-131, owned and operated by TWA, was receiving routine maintenance in Las Vegas, NV. During the check, the servicing valve on the forward water tank would not open. A TWA mechanic entered the forward electronics bay to check the valve control mechanism while the lead mechanic operated the valve from outside. The valve operated normally and the lead mechanic yelled "It's OK". A short time later, another mechanic looked in the compartment, did not see anyone and secured the hatch. The mechanic's absence was not noted until the next day. On 3-9-82, the airplane flew to St. Louis, MO where the mechanic's body was discovered after search was initiated by the Las Vegas station manager. An autopsy revealed that the mechanic was electrocuted when he came in contact with several 115 volt relays in the electronics compartment. Injuries found during autopsy indicate that the mechanic slipped and struck his head and fell on the relays.