From the evidence, the police think the boat was used to smuggle some 500 people from China into Japan two weeks ago. By now they have disappeared without trace - presumably into the lower ranks of Japan's labour market.
Try as they might, Japan's immigration authorities cannot keep up with the flow of illegal immigrants who are drawn to the country by the attraction of high wages. They come from China and Korea, from South- east Asia, from Bangladesh and from the Middle East.
They will work longer hours, for less pay, than Japanese workers. And, according to the Labour Minister, Masakuni Murakami, they are 'aggravating public ill-feeling'.
In April, Mr Murakami announced a new crackdown against illegal foreign workers. It was particularly aimed at Iranians, who had become conspicuous by holding a regular weekend market in a park in central Tokyo. The market was closed by the police, and shortly afterwards they began arresting and deporting Iranians who still lingered in the park.
At the same time police moved against South Koreans working in the construction industry and South-east Asian women working as hostesses in nightclubs. In one single blitz from April to May, more than 3,000 foreigners working illegally in Japan were rounded up and deported, according to the Justice Ministry.
As foreign labourers are firebombed in Germany and tossed around at the centre of a political storm in France, in Japan the government is simply kicking them out - if it can find them.
The Chinese are the most difficult to find. With physical characteristics similar to those of the Japanese, they can be easily absorbed into the already large population of Chinese living in the big cities of Japan. Police think that they are being brought in by the yakuza, organised crime syndicates, which already have jobs lined up for them when they arrive.
The Hosho Maru was registered in the name of a known yakuza member. In a similar incident last April, a ship that was intercepted off southern Japan with 149 Chinese on board was captained by a member of a yakuza gang. The Chinese were arrested on suspicion of attempting to enter Japan illegally.
The latest crackdown on foreign labourers has caused barely a ripple in Japan, where people are taught from a young age that a great gulf separates the 123 million Japanese from the rest of the world - usually referred to as gaijin, or aliens. Even opposition parties see little political advantage to be gained by attacking the government on immigration policy.
In fact, Japan feels even more vulnerable to a flood of immigrants than does Western Europe: the tens of millions of Eastern Europeans who covet the lifestyles of Western Europe pale beside the hundreds of millions of Chinese and South-east Asians who look longingly at Japan's disproportionate affluence. The government's answer to the income gap is to spend most of its foreign aid budget in Asia, hoping that economic improvement at home will dissuade other Asians from seeking jobs in Japan.
But despite generally rapid growth in Asia, stringent immigration checks at Japanese airports and harbours, and frequent crackdowns on illegal labourers in Japan, the immigrants keep coming. And there is plenty of work for them to do, particularly the dirty and low- paid jobs that the Japanese no longer care for.
According to the Justice Ministry, about 300,000 foreigners have overstayed tourist or student visas. Most are presumed to be working illegally. As fast as they can be deported, more come to take their places.