They said the crater glow intensified at about 7:50 p.m. Sunday, as observed from the Lignon Hill observatory and the Legazpi City proper.
"It coincided with a slight increase in the overall background tremor detected by seismographs around the volcano," the Philippine Institute of Volcanology and Seismology (Phivolcs) said in a statement.
The volcano, according to Phivolcs, has slightly inflated while the sulfur dioxide gas it emitted last week went beyond the daily baseline of 829 tons.
These factors, along with the intensifying crater glow and heightened seismic activity, indicate a new episode of unrest due to "magma intrusion at shallow depths."
Small explosions, similar to what occurred last June 3 and July 22, "may be expected as pockets of gas beneath the volcano’s crater are suddenly released," Phivolcs said.
Although alert level 2, which means a low to moderate unrest, is still hoisted, Phivolcs cautioned the public from venturing into the volcano’s permanent danger zone — a radius of six kilometers — because volcanic flows may occur anytime.
Last Sunday night, villagers panicked when super hot incandescent fragments rolled down from the Mayon summit and landed some three to four kilometers from the Bonga gully.
"These could be boulders that were heated by the continuous crater glow," said Phivolcs researcher Alex Baloloy.
Mayon is one of the country’s six most active volcanoes which volcanologists are closely monitoring. The five others are Taal in Batangas, Kanlaon in Negros, Bulusan in Sorsogon, Hibok-Hibok in Camiguin, and Pinatubo in Zambales.
Mayon last erupted in June 2001, causing the evacuation of more than 9,000 families.