He saw a man sitting inside an old Honda Civic, a vehicle that is a popular target for car thieves.
"You just get this sense," Williams said. "I took a look at the guy ... he sort of slouched down in his seat and pulled the cap down over his face."
It was suspicious that the man was trying to avoid eye contact with him even though he wasn't in his police uniform, Williams said.
Williams walked by the car and saw it was ransacked, which raised his suspicions even more. A trashed car is a sign that it might be stolen, since people are prone to taking good care of cars they've purchased themselves.
He called in the license plate and the car was in fact found to be stolen; the man, Wade Wolfe, 41, of Vallejo, had a fairly extensive criminal record.
"In this case, it worked out," Williams said.
Wolfe was arrested on charges of stealing the vehicle and possessing a hypodermic needle and a deadly weapon: He had a butcher's knife in the drivers' side door compartment.
The woman he was with, Shawn Bentley, 20, from Arizona, had a warrant out for her arrest and was also taken in. They were both booked at the Martinez Detention Facility.
While the lieutenant was waiting for backup to arrive at the scene, Wolfe started backing out of the lot. To avoid a car chase, Williams detained him at gunpoint.
Letting the suspect get on the road is about the last thing you want to let happen, he said.
"In a majority of these (stolen vehicle) cases a pursuit ensues, and that's not a good thing," he continued. "That definitely compromises public safety."
About eight officers showed up at the 7-Eleven, three from the Alamo Sheriff's Station and four or five from the Danville Police Department.
"These kinds of crimes can go sideways real quick and you always want to have a cover car there for a variety of reasons," said Williams. "Safety for the public, and our safety."
The car was stolen March 27 from the mall in Fairfield; a shaved key was used to start the ignition. Police have recovered four stolen vehicles in Danville so far this year.