Helen Goldsmith and Paul Heller noticed it several years ago on their regular walks around their neighborhood. It was a tiny golden triangle laid into the concrete at Sansome Street and the Embarcadero reading, “Roundhouse Plaza/Time Capsule Below/Brass Pin/To Be Opened June 30, 2018.”

“We looked down, and there it was,” Goldsmith said the other day as we stood over the triangle in question. “We thought, ‘That’s ages from now. We’ll figure it out. There will be news.’ And now, it’s now.”

Now it’s now, indeed. And Goldsmith and Heller, a married couple of 19 years, were no closer to solving the mystery. So they emailed me for help.

The triangular plaque has long sat in the corner of the plaza in front of Roundhouse One, also known as the Beltline Roundhouse, which was built in 1913 for trains running along the waterfront. It’s now a city landmark owned by the port and rented out as office space.

We met at the plaque to discuss the mystery.

“Who knows if anyone’s going to remember that it exists?” said Goldsmith, 59, a retired college administrator.

Heller, 64, a retired high school teacher, said the phrase “to be opened” implied that whoever buried the time capsule might not return to open it and was relying on someone else.

“I’m intrigued by who will show up to open it and how they’ll open it,” he said.

Asked what he imagined might be down there, Goldsmith paused in deep concentration. He said he pictured a metal box shaped like a triangle and filled with train-related items from the railroad days.

“What do you think is in it?” he asked his wife.

“I would like there to be copies of The Chronicle from whenever they buried this telling us what happened at the time,” she said, clearly a savvy woman to come up with that answer as a Chronicle columnist listened in.

Sadly, neither was right. Port officials decided to investigate so they would know whether to hold a ceremony revealing the contents of the mysterious time capsule.

“Before we created a suspenseful event and got everyone excited, we decided to check it out first,” explained Renee Martin, spokeswoman for the port.

On June 14, a port maintenance crew removed the plaque and dug down a couple of feet until it hit utility lines. And the workers found ... nothing.

No historic newspapers, no iron-horse memorabilia. No capsule of any kind.

The port is now deciding whether to bury an actual time capsule there, to be exhumed decades from now, or just cover up the hole.

Martin said the plaque was probably installed in 1983 when the plaza in front of the Roundhouse was redone. But why would someone install a plaque about a time capsule if there was no actual time capsule underneath?

“We have no idea,” Martin said. “Go figure!”

Martin said the port is leaning toward burying its own time capsule and is trying to figure out what to put inside it.

“Fifty years from now, what types of things might be interesting?” Martin asked. “Maybe some interesting things about the port and where we are with work on the seawall. Maybe an annual report.”

I think future capsule diggers would be mighty disappointed to find an annual report from the port.

A capsule should include paraphernalia more telling about life in San Francisco circa 2018. An iPhone X loaded with electric scooter apps? A $5,000 rent bill? A dirty needle? A “Resist” bumper sticker?

I do like Goldsmith’s idea of including copies of The Chronicle. They’d be rife with stories about the country’s president, a former reality TV star with no previous political experience, picking fights with Canada and separating immigrant parents seeking asylum from their little children.

Nah, nobody would ever believe that.

San Francisco Chronicle columnist Heather Knight appears Sundays and Tuesdays. Email: hknight@sfchronicle.com, Twitter: @hknightsf