Her family could only identify her from her motorcycle, a police spokesman said.
Elsewhere in the south three revenue department officials were wounded when a roadside bomb exploded as the van they were travelling in went past, police said.
Bombings and drive-by shootings occur almost daily in Thailand's Muslim-dominated southernmost provinces.
Since the violence flared in January 2004 more than 2,000 people have been killed.
Sonthi, who is the army chief, led the coup that ousted Thaksin Shinawatra as prime minister in September.
He was expected to meet troops to boost morale and discuss security issues ahead of the weekend's traditional New Year holiday when officials were instructed to be extra cautious.