"I thought, 'Oh gosh, a tortoise!'" she said.
Fearing the creature, which she described as "about the size of a small dinner plate," would soon become road kill, Thompson pulled over, emptied her shopping bag, scooped up the reptile and brought it home. Her 9- and 6-year-old kids Laila and Conrad quickly named it "Gilbert."
Oddly, the tortoise was wearing an electronic transmitter taped to its shell.
Thompson sent out an email to her Willows neighborhood group and received a response from a couple with a 60-pound tortoise of their own. Back in June they'd noticed a "lost turtle" sign and taken a photo, which they sent to Thompson. Sure enough, the poster for the missing Russian Tortoise fit "Gilbert's" description:
"Small tortoise missing from north Palo Alto backyard near San Francisquito Creek: Seven inches across, brown with greenish tinge, very friendly; locator tag taped to shell with blue masking tape." A $200 reward was also offered.
Thompson had removed the tape and transmitter from the tortoise's carapace but was intrigued as to its purpose. Friends jokingly suggested it might be involved in some sort of espionage ring, with Thompson wondering, "Am I in a Stieg Larsson novel here?"
She tracked the poster (and a corresponding Craigslist ad) to Stanford University professor Ursula Heise and sent her an email.
Though Heise was on a month-long trip to Sweden, she responded immediately.
"Gilbert," it was revealed, is actually a female called "Empty Fox" (named after a character in a 1960s novel) — one of three beloved pet tortoises kept by Heise. The mysterious locator tags are used by Heise to more easily keep track of the tortoises when they're outdoors but despite her regular combing of the neighborhood with the receiver, she could not locate Empty Fox's signal after she escaped from the enclosed yard.
When contacted by Thompson, "She was over the moon," Thompson said of Heise. "After three months she'd slowly given up hope. She was thrilled."
Heise concurred. "I just cried; I was so happy," she said.
Thompson agreed to tortoise-sit Empty Fox for the week until Heise's return from abroad. On Monday, Empty Fox was reunited with Heise — and her tortoise sisters — amid much hugging and celebrating by the humans involved.
"She is such a sweet little tortoise, so tame. The kids wanted to keep her; they were shedding tears, but we already have 10 times too many pets," Thompson said. Her menagerie includes a bearded dragon, a parrot, a dog, rabbits and a bullfrog.
Plus, despite her fondness for the tortoise houseguest, she said reuniting her with Heise was the right thing to do.
"I know how I'd feel if our bearded dragon got out, as a fellow reptile lover," she said. And once her kids saw how happy Heise was, they "snapped out of their 'finders, keepers' mentality."
Though Empty Fox traveled only a few blocks from home during her three-month absence, what she was up to during that time remains a mystery. Heise suspects she simply "dug in" — burying herself underground, something she's fond of doing at home — for most of the time. Last week's warm temperatures may have drawn her out of hiding.
"She didn't go that far considering she was gone for a little over three months. They can walk at a good clip. As opposed to the stereotype, they can walk around pretty quickly if they put their minds to it.
"She's back with the two other tortoises, and we're really, really happy to have her back," Heise said.
If Empty Fox has any thoughts on her adventures or her return, she's keeping them to herself. Granted, "It's hard to tell with a tortoise," Heise said. "Right now she's just spending a lot of time out in the sunshine and inside, under her heat lamp."
Thompson declined the offered reward so Heise is instead donating $200 in her name to Mickaboo Companion Bird Rescue, which helps pet birds in need of adoption and fostering.
"Holly was just incredibly generous, saying, 'This is what anybody who loves animals would do'," Heise said.
Added Thompson: "It was exciting for me to find someone's lost love and reunite them."