The Oslo express is racing across the pre-dawn, frozen pine forests of Norway, snow shawling over the corner window of carriage three, wherein sits Detective Inspector Fisk on the last day of “Operation Aleppo”. He is reading Rafael de Nogales’ Memoirs of a Soldier of Fortune, a 1932 edition of the Venezuelan general’s account of derring-do in revolutionary Latin America and service in the First World War Ottoman army.

On page 294, he meets a doomed German soldier in the Middle East, a “tall, handsome young officer” who has disgraced his Prussian honour by having an affair with a girl who claimed she was 18 when in fact she was only 16.

And oddly – for books often carry weird geographical hints about ourselves – General Nogales briefly meets this young man in the German military mess “sometime in August 1915, when I arrived at the city of Aleppo, after six months’ steady fighting against the Russians and Armenians in the Caucasus”.

Odd. For Inspector Fisk is investigating events that occurred in that same city of Aleppo more than 90 years later, when a Syrian army general on the city’s front line ordered his soldiers – just three months ago – to show me weapons recently captured from the country’s anti-Assad resistance.

And among the grenades, rifles and explosives, was a plastic packet containing three pink sticks of what looked like gelignite, on each one of which was labelled “Hammargrens, 434-24 Kungsbacka, Sweden”.

Kungsbacka is a small town south of the Swedish city of Gothenburg, which was where my night train was now thundering – the reader has just grasped why The Independent’s Middle East Correspondent is more than 2,000 miles from his base in the Levant – but, like every good detective story, there’s always a twist.

With Detective Inspector Lewis and Detective Sergeant Hathaway, you’ll always find that things are not quite what they seem. So it was with some excitement that as my taxi from Gothenburg arrived on the edge of a sleet-swept forest at Kungsbacka, I observed a steel security gate with the name “Hammargrens Pyroteknik” upon it, behind which stood a concrete explosives bunker.

Hammargrens, I should add, sells children’s fireworks. And Thomas Wetterstrom, its managing director, looks the part. Bespectacled, swarthy, 60 years old, 30 years with the company, which old man Hammargrens founded in 1879, he peered at the photograph of his product in far-away Aleppo with what I can only describe as wry amusement. “These are warning flares,” he said. “I don’t really see what the Syrians can do with them. We sell them to the Swedish police to slow down traffic after accidents.”

The Swedish police? This was a bit close to home for Inspector Fisk (and “Fisk”, by the way, means “fish” in Swedish, for my ancestors indeed came from Scandinavia in 1745) – but it was perfectly true.

Mr Wetterstrom imports the flares from an American company called Orion Safety, in Maine (which accounts for the “made in USA” stamp I observed on the Aleppo batch), and then sells them not only to the Swedish police, but for ambulances that attend night-time road accidents in the Scandinavian tundra. Hammargrens had also exported flares to the Hungarian police. Like our British flashing safety lamps, they prevent other motorists careering into the police and paramedics on motorways.

Don Kantoff, Hammargrens’ explosives officer, a thin, precise man – as are almost all explosives experts, I’ve noticed – asked me if I’d like a demonstration. So we padded out into the sleet; he struck the cap on the top of the flare and it burst into an astonishing pink flame, so bright that I could only glance at it for a second before it hurt my eyes. “It lasts for 20 minutes,” Mr Kantoff announced. “And it cannot be put out by water.”

This was no doubt efficient on snow-covered Swedish roads. But I also had a suspicion that, thrown into one of Aleppo’s ancient wooden buildings on the city’s front line today, it would set the place ablaze.

Mr Kantoff reckons that the company had sold 100,000 of these flares over the past 12 years. They come in cartons of 23, each containing a pack of three, a mixture of strontium carbonate and sulphur that burns down to ash – a home-made bomb in Syria would need potassium nitrate and oil – although all at Hammargrens agree that any chemicals, if put inside an iron tube, are dangerous.

But as Mr Kantoff studies the Aleppo picture, he notices the plastic packet: “We stopped using these plastic packets years ago, before we sold to Hungary.” He hands me the company’s up-to-date cardboard box.

It turns out that back in 1999 – the date on the Aleppo flares – Hammargrens was selling flares to a Stockholm company which supplied them, along with emergency blankets and bandages, to the big Swedish lorry-makers, Skania and Volvo.

And at this point, Inspector Fisk remembered a lecture he attended in Abu Dhabi four years ago, in which a Swedish diplomat boasted that Volvo was the biggest exporter of trucks to – Syria! Volvo had cornered the market – in pre-civil war Syria, of course – and there was no legal reason why these trucks should not have been sold to Syria with Hammargrens’ flares in them.

The twist in the story. Everything above board, all sold for safety. But a thought occurs to Inspector Fisk – a “Hathaway moment” from the Lewis series, shall we call it? – on the night train back to Oslo. Sweden hasn’t endured a war since 1814. But shouldn’t we Europeans be a little more careful what we send to less stable parts of the world? Fewer flares, perhaps? More of those old-fashioned, battery-powered British safety lamps?

So on my super-heated train home, I turn again to my soldier of fortune, General de Nogales.