Machado thinks the sign was shot recently and that it might be a clue.
She also tries to soothe the nerves of angry local ranchers, one of whom suffered a heart attack after a cow was gunned down in his field.
"Hey, we all lose animals; they die. But these killings are off the charts," said Mike Gale, president of the Marin County Farm Bureau. "They've gotten under the skin of the ranching community here. If they catch these kids, I'm afraid they're going to do something terrible to them."
Machado knows the anger. She once ran a cattle ranch. Cows are more than just walking slabs of beef, she says. You get attached to them, name them.
Photos of the crime scenes decorate the walls of Machado's office. She scours the shoulders of roads, using a metal detector to hunt for spent shell casings.
She brought one rotting calf carcass to the shelter where she works to X-ray it for metal fragments: "It was looking for a needle in a haystack. But we had to try."
Officials have offered a $16,000 reward in the Northern California cattle cases. But so far, it has yielded no leads.
Machado isn't giving up. One afternoon, she leaned out of her patrol truck and offered her card to two girls and a boy who were feeding cattle at their family's ranch. She drove away, observing the teens in her rearview mirror.
"The kids killing these animals are out here somewhere," she said. "I hope we find them soon. They really need help."