But such was the heavy police presence for the guest’s departure it left onlookers to guess it could only be Kim, on his first visit to Beijing since taking power in 2011.

Earlier, the evidence was compelling, someone big was in the capital, and they came direct by train from North Korea.

But who?

In a smartphone age, complete secrecy is impossible. The digital trail began when images hit social media on Monday of cladding erected to shield a train passing from North Korea at the Chinese border town of Dandong.

Photographs were posted as the train rolled southwards – was this the same green armoured train that had previously carried Kim Jong-il, the father of the current North Korean leader?

Video captured the train entering Beijing, something special because police were blocking the streets running parallel to the rail line.

Japanese TV broadcast shots of an honour guard of Chinese soldiers greeting its arrival.

Another video showed a motorcade speeding off from Beijing train station, escorted by 23 motorcycle outriders.

Trains on Beijing's subway were bypassing the central station. Tiananmen Square was closed to tourists.

The next morning, multiple fire trucks were parked throughout the embassy district. Was this increased protection for the rest of the world's diplomats because a train load of North Koreans had arrived it town?

Was it even, as rumoured, North Korean leader Kim Jong-un himself making his first trip to Beijing since taking office seven years ago?

And was he there to meet Chinese President Xi Jinping – also as rumoured?

The bleak North Korean embassy was circled by Chinese plain clothes police, all equipped black with earpieces, scanning the street.

But this was not where the mystery guest was staying.

Across town, Fairfax Media saw the perimeter of the Diaoyutai State Guest House swarming with hundreds of police and soldiers standing to attention every 10 metres.

Along Beijing's Changan boulevard, stretching west from Tiananmen Square, PLA sentries stood on pedestrian bridges, in clusters at every street corner and bus stop, in preparation for a motorcade. Then it streaked past.

South Korea's presidential office confirmed a high ranking North Korean official had entered China. It was watching, but did not yet know the official's identity.

South Korea saw it as a positive sign that China and North Korea were seeking to improve relations ahead of the summit between the two Koreas in April, and Kim and US President Donald Trump's meeting in May.

South Korean analysts speculated it could be Kim's sister, Kim Yo-jong, or Kim Yong-nam, North Korea's ceremonial head, who also attended the Korean Olympics and met with South Korean President Moon Jae-in, the highest level contact between the Koreas in a decade.

Zhang Liangui, a former professor with China's Central Party School, told Fairfax Media it was a "good trend" if North Korea had sent a senior official to Beijing.

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"In the past, many believed that the North Korean nuclear issue was a matter between North Korea and the United States, and China was not the party involved. As a result, China was excluded from the scope of the dialogue."

"Now everyone has began to realise that the North Korean nuclear issue is not a bilateral issue between North Korea and the United States, but a regional security issue. All countries in the region, China, South Korea, Japan, and Russia, plus the United States, should participate."

If past practice on visiting North Koreans is a guide, the identity of Beijing's guest won't be revealed until after they have left.