Searing heat clouds shot out from the volcano in Java late on Wednesday and experts warned that it remained highly dangerous.
Days earlier thousands of villagers had left their homes fearing the worst, but reduced activity following major emissions of gas and ash on Monday seemed to indicate a return to calm.
Volcanologists monitoring the 3,000-metre peak say the eruption process is still continuing and they cannot predict whether it will get worse or calm down.
Overnight on Tuesday, four heat clouds shot out from the mountain, travelling up to 3.5km down its slopes. At least two were recorded later in the day, with one stretching down four km.
Scientists warn that the larger and more deadly heat clouds are typically preceded by smaller ones such as those of the past few days.
Clouds covered the volcano's peak on Wednesday afternoon, making it difficult to see any fresh lava flows.
Life goes on
More than 22,000 residents evacuated from the immediate area of the volcano are either waiting in camps or have returned to their homes under surveillance by authorities.
At one camp an official from the disaster control post said 131 people had arrived in the camp on Tuesday.
"Because of the eruption of the heat clouds on Monday, we did not even have to beg them to come down. They came down on their own," the official, named Suyadi, told AFP.
Makeshift refugee camps are
virtually empty during the day
But on Wednesday morning the camp was virtually empty.
"It's just as usual. They all attend to their own chores at home" during the day, he said.
Many residents have been returning to their homes during the day to collect grass and feed their livestock.
Scientists said on Tuesday that the new lava dome forming at the peak of Merapi contained some 2.3 million cubic metres of lava with 150,000 cubic metres being added daily.
The main fear, they say, is that the dome may collapse and shoot out blazing lava as well as more deadly heat clouds.