It's no secret most countries around the world are watching each other.
China gave a brazen reminder of this when one of its suspected spy balloons traversed US airspace — before being shot down — last week.
The balloon captivated public attention for being a seemingly peculiar vehicle for espionage.
But the US has since said China has used such surveillance balloons to target more than 40 countries on five continents.
NATO chief Jens Stoltenberg warned this week there had been increased Chinese intelligence activities in Europe, citing the use of satellites and cyber threats.
Meanwhile, an audit found that Chinese government-linked equipment was found in hundreds of Australian Commonwealth buildings, including defence and foreign affairs offices.
It prompted urgent calls for a plan to "rip" the hundreds of devices out of government sites.
But in the vast and ever-evolving world of espionage, intelligence experts say China's tactics aren't particularly advanced.
The Pentagon has also insisted the spy balloon did not give China an intelligence collection capability above what it already has via satellites and other means.
So what other techniques and technologies are countries using to snoop on each other, and what's proving valuable in a world of rising tensions?
The power of 'obvious' open source
The intelligence cycle never slows down, John Blaxland from ANU's Strategic and Defence Studies Centre, and author of Revealing Secrets, told the ABC.
Whether in peace or war, analysts are constantly on the lookout for what another country's capabilities are, what they are declaring they can do, and how those intentions may be carried out.
"You're wanting to collect intelligence on emerging technologies, emerging tactics and techniques," Professor Blaxland said.
"We know that they've got drones, we know they've got these old tanks, but are they coming up with innovative new ways of using them?"
Information collection comes in various forms, whether it's through "good old-fashioned" spies on the ground, signals, imagery, or cyber intelligence.
But Kristian Gustafson, from Brunel University London's Centre for Intelligence and Security Studies, said open source was where countries like the UK, US and Australia were increasingly honing their efforts.
"We focus on the major, big things like a spy balloon, Earth observation satellites, or various types of space-based intelligence collection," he told the ABC.
"But variously, 70 to 90 per cent of usable intelligence for any state is based on open source material."
Open source isn't necessarily hacked information, it often comes directly from what is published.
There are sophisticated ways of analysing open information to get a sense of what a country is developing in regards to artificial intelligence (AI), hypersonics, or space-based observation, Dr Gustafson said.
"Weapons development is based on pure research. It's pure research that then gets weaponised," he said.
"It's so obvious it's often overlooked."
All eyes on Ukraine
The power of open-source information has been on display in Ukraine.
Smartphone geolocation and high-res civilian and commercial satellite images from Maxar and Google have dramatically increased the volume and accessibility of valuable intelligence.
"Not only have we multiplied the amount of Earth observation, but we've made it accessible to a massive number of people," Dr Gustafson said.
For instance, in the lead-up to Moscow's full-scale invasion, imagery from Maxar and collected social media posts portrayed a very public build-up of Russian forces.
Although they tried, It made it difficult for Russia to refute its intentions.
"It makes traditional kinds of denial and deception — the normal practice of trying to hide what you're doing from an adversary — much harder," Dr Gustafson said.
"Russia hasn't caught up with the open source revolution."
Mobile signals have also been used to carry out major attacks.
Russia's defence ministry blamed the illegal use of mobile phones for a Ukrainian missile strike that killed 89 servicemen on New Year's Eve.
"The ubiquity of the mobile phone is a phenomenally powerful and phenomenally dangerous device," Professor Blaxland said.
"It's incredibly enabling and incredibly destructive if you're foolish enough to post your whereabouts when you're in range of enemy artillery."
China 'a different kettle of fish'
Collecting open-source information becomes harder when you're looking at a country like China because of its "great firewall", Dr Gustafson said.
It tries to cut off that line of intelligence by restricting and policing things like social media usage.
"The Chinese are a different kettle of fish," Dr Gustafson said.
"They're much more like what Russia would have been like during the Cold War.
"They're a hard target, or certainly a much harder target."
Technological competition and counterintelligence have therefore been at the centre of the "Five Eyes" approach to China.
Five Eyes is the intelligence alliance between Australia, Canada, New Zealand, the United Kingdom and the United States.
"ASIO, the MI5 security service in the UK, and the FBI are turning much more forcefully to this counterintelligence role against adversaries who are trying to steal research and primary industrial information that provide us a technological edge," Dr Gustafson said.
US officials are in private talks about the fate of TikTok, which is owned by Beijing-based ByteDance, over concerns China could use the app to collect data on its users.
There have been actions taken against Chinese tech giant Huawei over allegations it has the capacity to facilitate spying.
In 2018, Huawei was banned from providing equipment to Australia's 5G network.
The US also passed a sweeping set of regulations in the last year aimed at kneecapping China's semiconductor industry.
The 'thousand grains' approach
Professor Blaxland said China aimed to collect information on a large scale in what he described as the "thousand grains of sand approach".
"If everybody just collected one grain of sand from a beach, and you get 1,000 grains of sand, then you'll get a good idea of what kind of sand there is at that beach," he said.
"Chinese espionage operates on an industrial scale, they collect information routinely and they're doing it intrusively."
However, the intelligence space is extremely dynamic and China is lagging behind in innovation.
"As soon as one new technique or technology is discovered and emerged, a countermeasure is in the works," Professor Blaxland said.
"China is very good at following trends, it doesn't tend to be leading the trends.
"They are looking to copy the latest and greatest."
Generally, it has its eyes on Israel, which is renowned for its tech-savvy intelligence.
For instance, its Pegasus spyware, which is considered the "world's most powerful cyber weapon".
"Israel is probably — other than Ukraine — the most innovative when it comes to the kind of spyware and cyberware they have," Professor Blaxland said.
"But Israel sometimes has an incentive to try and make money by selling it to other countries, China included."
Human contact and contraptions
Coat button cameras and cats being used to spy on Kremlin and Soviet embassies were among the many creative tactics used during the Cold War.
Professor Blaxland said earlier on in the digital age, there was a sense that analog spy kits would all be relegated to the museums.
But with digital footprints being so pervasive and traceable, they have, to an extent, come back into vogue.
"There's still human-to-human intelligence trade craft, absolutely," Dr Gustafson said.
"That's the stuff that you're going to read the absolute least about, because that is the most sensitive."
There were reports last year that former British prime minister Liz Truss's personal mobile was hacked by suspected Russian agents when she was foreign minister.
Dr Gustafson said that kind of hack would have required some sort of human access.
"It's not someone breaking into your system, it's one of your people accidentally, or purposefully, letting the adversary in," he said.
While this sort of human trade craft still exists, it comes at a high cost, with high risk and long-term investment for relatively low volume, he added.
Analysis and the AI contest
While there are various forms of intelligence collection, the art of espionage often comes down to analysis.
"You can hoover up everything, but somewhere along the line a human has to think, 'Is this of any consequence?'" Professor Blaxland said.
"The overwhelming majority of data that's collected isn't looked at, and is usually dispensed shortly afterwards."
The right analysis helped Ukraine prepare for Russia's full-scale invasion, Dr Gustafson said.
It wasn't the images on their own, it needed the work of sophisticated analysis of Russian moves.
This is where AI will be the game changer.
"The bottleneck is always going to be in the humans adding value to that information," Dr Gustafson said.
The contest is on to be able to preload an autonomous system with analysis, so it can interpret what it sees at the same level of human intelligence, he added.
"The first state that is going to run a general AI, to own a general AI, it's going to be like commissioning the Dreadnought.
"It's going to make everything else irrelevant the moment that you deploy an AI."
