It's nearly midnight in Singapore.

A senior officer of the Luftwaffe, the German Air Force, is in his hotel room.

He's in the region to rub shoulders with defence industry players at Asia's largest air show.

He has had a long day - but he can't go to bed just yet.

Brigadier General Frank Gräfe has a work call to dial into with his boss - the commander of the German air force.

It's not a big deal for the head of Air Force Operations. He sounds relaxed on the line as he chats with two colleagues about the "mega" view from his room, and how he's just come back from a drink at a nearby hotel where there's an incredible swimming pool.

"Not too shabby," one of them remarks.

Finally, the boss, Lieutenant General Ingo Gerhartz, dials in - and they begin. Over the next 40 minutes, the group appear to touch upon highly sensitive military issues, including the ongoing debate over whether Germany should send Taurus cruise missiles to Ukraine.

What none of the call's participants know is that they're being eavesdropped on - and their conversation is being recorded.

Two weeks after the call took place, the audio tape was leaked by Russia's state-run RT channel.

Germany hasn't said whether they believe the recording may have been tampered with - but they have confirmed that the call did take place and that it was intercepted by, they believe, Russian spies.

Their man in Singapore had, according to the German government, sprung "a data leak".

While he hasn't been officially named, it's implied that it was Frank Gräfe who accidentally let spies onto the call.

Soon, their supposedly top-secret discussion spilled out via Russian state media and echoed across the world.

The apparent contents of the call are now well-known.

The four participants discussed what targets German-made Taurus missiles could potentially hit if Chancellor Olaf Scholz ever allowed them to be sent to Kyiv - a contentious issue in Germany.

French and British weapons deliveries were also brought up, including the highly sensitive suggestion that a "few" British personnel are allegedly on the ground in Ukraine.

But how were spies able to eavesdrop?

The answer we've been given so far boils down to a case of human error.

According to German authorities, the "data leak" was down to just one participant dialling in on an insecure line, either via his mobile or the hotel wi-fi.

The exact mode of dial-in is "still being clarified", Germany has said.

"I think that's a good lesson for everybody: never use hotel internet if you want to do a secure call," Germany's ambassador to the UK, Miguel Berger, told the BBC this week. Some may feel the advice came a little too late.

Eyebrows were raised when it emerged the call happened on the widely-used WebEx platform - but Berlin has insisted the officials used an especially secure, certified version.

Professor Alan Woodward from the Surrey Centre for Cyber Security says that WebEx does provide end-to-end encryption "if you use the app itself".

But using a landline, mobile phone, or open hotel wi-fi could mean security was no longer guaranteed - and Russian spies, it's now supposed, were ready to pounce.

Professor Woodward says that spies were "probably sitting on the fringes of the Singapore Air Show".

The biennial event, which this year took place on 20-25 February, typically attracts high-level government, military and industry figures.

If you're a spy, "when you get gatherings like that, it's always worth sitting in the car park or getting a hotel room", says Professor Woodward.

The Russians could have, theoretically, used long-range antennae combined with computer programming capable of capturing local network traffic.

"Essentially these intercepts are like rattling door handles and seeing what you can find," Professor Woodward says. "Eventually you find one that's unlocked."

A researcher in cryptography in Berlin, Henning Seidler, believes the most likely theory is that the officer dialled in via his mobile phone and the call was picked up by spies' antenna who can also "forward" the traffic onto the main, official antenna.

But all the while, "they are just listening and writing down everything that's being transmitted".

"It's like fishing with dynamite. You just throw a stick of dynamite in a pond and see which fish are floating up afterwards."

"This was their most juicy catch."

Berlin was anxious to rule out one theory that was doing the rounds - that a Russian spy simply dialled in and sat on the line, without anyone noticing.

And the government is insisting that, while they are investigating what happened, this is all essentially down to one man's mistake.

The call was netted in a widespread fishing exercise, they argue. The spies got lucky, while Germany didn't.

Former senior army officer and Bundestag member, Roderich Kiesewetter, is among those who don't quite buy the "this could have happened to anyone" line of defence.

"You have to choose a certain kind of disguise for this disaster," says Mr Kiesewetter, who's also worked at the Nato military alliance and is a member of Germany's opposition conservative CDU party.

He believes that a "peacetime" mindset has allowed complacency to set in.

"It might be a personal mistake," Mr Kiesewetter says. "However, it is a signal of a systemic failure."

He also believes Germany is a "soft target" due in part to a "widespread Russian romanticism" dating back to World War Two.

But German government figures find suggestions that they are somehow soft on Russia increasingly irritating, particularly because Berlin has donated more weapons aid to Ukraine than any other nation in Europe.

Ministers also believe that Moscow deliberately released the leaked tape on the day of opposition leader Alexei Navalny's funeral in a deliberate attempt to distract at home and divide abroad.

Russian President Vladimir Putin's playing a "perfidious" game that "we must not fall for", said Defence Minister Boris Pistorius this week.

Russia has neither confirmed, or denied, that its intelligence service was behind the hack.

But whoever it was that picked up an insecure line in a Singapore hotel room late one February night, this Luftwaffe leak has been damaging for Germany.

It's further exposed domestic divisions about whether to send Taurus missiles to Ukraine and prompted a wider discussion about the country's perceived defence and security weaknesses.

In Berlin, they're just hoping that the leak was, indeed. just a one-off - rather than the tip of the iceberg.

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