Piew Balang, 70, a Lahu woman, went to collect bamboo shoots in Chaeson National Park on Thursday and did not come out of the forest.
Her relatives sought help from the authorities, who formed teams to search for her on Friday into the evening but could not locate her.
Pichia Watcharawongpaiboon said officials found picked bamboo shoots and evidence that a person had slept on the ground at a spot in the park.
Officials expect to locate her soon, said Pichia.