Or so dozens of unsuspecting passengers thought.

The curly-haired man tried to keep to himself, intently if inscrutably scribbling on a notepad he’d brought aboard. His seatmate, a blond-haired, 30-something woman sporting flip-flops and a red tote bag, looked him over. He was wearing navy Diesel jeans and a red Lacoste sweater – a look he would later describe as “simple elegance” – but something about him didn’t seem right to her.

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She decided to try out some small talk.

Is Syracuse home? She asked.

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No, he replied curtly.

He similarly deflected further questions. He appeared laser-focused — perhaps too laser-focused — on the task at hand, those strange scribblings.

Rebuffed, the woman began reading her book. Or pretending to read, anyway. Shortly after boarding had finished, she flagged down a flight attendant and handed that crew-member a note of her own.

Then the passengers waited, and waited, and waited for the flight to take off. After they’d sat on the tarmac for about half an hour, the flight attendant approached the female passenger again and asked if she now felt okay to fly, or if she was “too sick.”

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I’m OK to fly, the woman responded.

She must not have sounded convincing, though; American Airlines flight 3950 remained grounded.

Then, for unknown reasons, the plane turned around and headed back to the gate. The woman was soon escorted off the plane. On the intercom a crew member announced that there was paperwork to fill out, or fuel to refill, or some other flimsy excuse; the curly-haired passenger could not later recall exactly what it was.

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The wait continued.

Finally the pilot came by, and approached the real culprit behind the delay: that darkly-complected foreign man. He was now escorted off the plane, too, and taken to meet some sort of agent, though he wasn’t entirely sure of the agent’s affiliation, he would later say.

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What do you know about your seatmate? The agent asked the foreign-sounding man.

Well, she acted a bit funny, he replied, but she didn’t seem visibly ill. Maybe, he thought, they wanted his help in piecing together what was wrong with her.

And then the big reveal: The woman wasn’t really sick at all! Instead this quick-thinking traveler had Seen Something, and so she had Said Something.

That Something she’d seen had been her seatmate’s cryptic notes, scrawled in a script she didn’t recognize. Maybe it was code, or some foreign lettering, possibly the details of a plot to destroy the dozens of innocent lives aboard American Airlines Flight 3950. She may have felt it her duty to alert the authorities just to be safe. The curly-haired man was, the agent informed him politely, suspected of terrorism.

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The curly-haired man laughed.

He laughed because those scribbles weren’t Arabic, or another foreign language, or even some special secret terrorist code. They were math.

Yes, math. A differential equation, to be exact.

Had the crew or security members perhaps quickly googled this good-natured, bespectacled passenger before waylaying everyone for several hours, they might have learned that he — Guido Menzio — is a young but decorated Ivy League economist. And that he’s best known for his relatively technical work on search theory, which helped earn him a tenured associate professorship at the University of Pennsylvania as well as stints at Princeton and Stanford’s Hoover Institution.

They might even have discovered that last year he was awarded the prestigious Carlo Alberto Medal, given to the best Italian economist under 40. That’s right: He’s Italian, not Middle Eastern, or whatever heritage usually gets ethnically profiled on flights these days.

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Menzio had been on the first leg of a connecting flight to Ontario, where he would give a talk at Queen’s University on a working paper he co-authored about menu costs and price dispersion. His nosy neighbor had spied him trying to work out some properties of the model of price-setting he was about to present. Perhaps she couldn’t differentiate between differential equations and Arabic.

Menzio showed the authorities his calculations and was allowed to return to his seat, he told me by email. He said the pilot seemed embarrassed. Soon after, the flight finally took off, more than two hours after its scheduled departure time for what would be just a 41-minute trip in the air, according to flight-tracking data.

The woman never reboarded to the flight.

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Casey Norton, a spokesman for American Airlines (whose regional partner Air Wisconsin operated the flight), said the woman had indeed initially told the crew she was sick, but when she deplaned she disclosed that the reason she was feeling ill was her concern about the behavior of her seatmate. At that time, she requested to be rebooked on another flight. The crew then called for security personnel, who interviewed Menzio and determined him not to be a “credible threat.” Norton did not know whether the woman was ever notified that Menzio had been cleared. (He said he was not allowed to give out her name for privacy reasons, and since Menzio did not know it either, I have not been able to contact the woman for comment.)

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Whenever there are conflicts between passengers, Norton said, “we try to work with them peacefully to resolve it,” whether that means changing seat assignments or switching someone to take a different flight. When asked how often customers raise similar suspicions about fellow passengers that turn out to be unfounded, he said it happens “from time to time” but declined to provide details about frequency.

Menzio for his part says he was “treated respectfully throughout,” though he remains baffled and frustrated by a “broken system that does not collect information efficiently.” He is troubled by the ignorance of his fellow passenger, as well as “A security protocol that is too rigid–in the sense that once the whistle is blown everything stops without checks–and relies on the input of people who may be completely clueless. ”

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Rising xenophobia stoked by the presidential campaign, he suggested, may soon make things worse for people who happen to look a little other-ish.

“What might prevent an epidemic of paranoia? It is hard not to recognize in this incident, the ethos of [Donald] Trump’s voting base,” he wrote.