Ash fell on the town of Blitar in East Java, which is located outside a 10-kilometre danger zone around the peak of Mount Kelut, but officials say it came from a second volcano further away from the town.
Residents initially feared the worst when the ash drifted over the town, Blitar district spokesman Sukamtono said.
“We have checked with the volcano monitoring team, and they say that the ash does not come from Kelut, but from Semeru,” he said, referring to another volcano about 90 kilometres away.
Semeru has been spewing ash on and off for months and scientists say that the volcano's activities were still considered normal and not dangerous.
Meanwhile scientists said they were baffled by the behaviour of Kelut, with the energy surging inside it surpassing that of its last eruption, in 1990.
Scientists abandoned their monitoring posts on Kelut's slopes on Saturday when tremors became so strong they could no longer be measured.
They could not see the peak through heavy cloud but said they believed it was erupting.
However they later found that it had not.
Volcanologist Agus Budianto said that pressure inside Kelut was three to four times as strong as that which caused the last eruption, which killed 34 people.
That eruption created a blockage that magma has not been able to fully break through, lifting only some volcanic material under the crater lake and resulting in a column of steam rising from its surface, he said.
“All indications point to an eruption, but the fact is, there has so far been only a partial lifting of the lava dome at the top, as well as a strong drift of heated winds upwards,” Mr Budianto said.
The lifting of a portion of the crater to the south-southwest – the direction evacuation efforts have focused on — has resulted in water mixed with volcanic material gushing down the usually dry Bladak river bed.
Cracks resulting from the crust movements were providing an escape route for magmatic gases, he said, gradually reducing the pressure inside – and meaning that it was possible no eruption would occur.
Underground tremors however were ongoing, he said, while the temperature of the crater lake has soared to 77.5 degrees Centigrade, compared to 40 degrees when it erupted in 1990.
“This is something entirely new for Kelut,” Budianto said.
A group of journalists who sneaked to the edge of the crater on Monday said that the lake had turned whitish and bubbles were forming every two minutes or so on its surface. A strong sulfuric odour also permeated the peak, though birds remained in the area, they said.
Authorities sealed off the mountain and tried to evacuate all 130,000 people living in the danger zone since it was put on high alert on October 16.
Many however have either refused to leave or returned to their homes and fields during the day to work.
Since records Mount Kelut has claimed more than 15,000 lives, including an estimated 10,000 in a catastrophic 1586 eruption. A 1919 eruption spewed heat clouds that killed 5,160 people.
Indonesia sits on the so-called Pacific “Ring of Fire” where several continental plates collide, causing frequent seismic and volcanic activity.