"That is part of the problem -- we don't know what is released," Benicia Mayor Elizabeth Patterson said in an email Tuesday.
Valero is not releasing detailed information about Sunday's malfunction, but it is linking it with the problem earlier in the month that sent flames, smoke and toxic gas into the sky.
"Valero's Benicia refinery continues to work to restore operations following the May 5 power outage caused by disruption from Pacific Gas & Electric," said company spokeswoman Lillian Riojas in an email almost identical to one she sent to KQED two weeks ago about another release.
"Intermittent flaring and related startup issues may occur," Riojas said.
The problem was tied to Valero's scrubber unit, city officials said.
That unit acts as a large filter, cleaning up gas or liquid that exits a processing unit, according to Tulane professor Eric Smith, who specializes in gas and oil production.
The refinery sent black smoke that came from oil residue into the sky on three occasions Sunday afternoon, each one lasting between 15 and 45 seconds, said Benicia Fire Chief Jim Lydon.
The air district then sent an inspector to the facility, according to Tom Flannigan, a spokesman for the Bay Area Air Quality Management District.
"This new incident that happened over the weekend may have been tied to them testing out their equipment and their processes as they bring them back up to speed," Flannigan said in an interview Tuesday.
The district has already issued six notices of violation against Valero in connection with the May 5 outage.
"We did see some dark black smoke that was released for a period of time," Flannigan said. "We'll be taking a close look to see what exactly what was emitted and if they're subject to any fines."
Unlike a series of releases that took place at the refinery in the days after the outage, Sunday's incident was not reported to the Governor's Office of Emergency Services, which administers a hazardous materials notification database. You can see the Valero outage notifications here.
In fact, state air regulators were not kept in the loop either.
"We have nothing new, and nothing new was reported to Cal OES over the weekend," said David Clegern, a spokesman for the California Air Resources Board, which has had air monitors in place near the refinery since the outage.
The outage is under investigation by a third party hired by PG&E, the air district, California's Division of Occupational Safety and Health (Cal/OSHA) and Solano County's Department of Resource Management.
The county is investigating another incident that may be tied to the aftermath of the outage. On May 15, several people who work at MRC Global, a company on Bayshore Road not too far away the refinery, found an "oil-based" substance on their cars.
A hazardous materials specialist conducted tests on the vehicles, according to Terry Schmidtbauer, director of Solano County's Department of Resource Management.
City and county officials asked Valero to take the vehicles to a car-wash, Schmidtbauer said.