(UPDATE: They say it was most likely radium-226, the surface radiation was 600 microsieverts/hour.)
From Tokyo Shinbun quoting Kyodo News (10/14/2011):
Regarding the localized hot spot location on the municipal road in Tsurumaki, Setagaya-ku in Tokyo, the Mayor of Setagaya-ku, Nobuhito Hosaka, held a press conference on October 13 evening that over 30 microsieverts/hour radiation was detected from the bottles found from under the floor of the house that borders the road.
The Ministry of Education and Science analyzed the content of the bottles, and almost identified it as radium. Since it was not one of the nuclides released from a nuclear reactor such as radioactive cesium, the Ministry concluded that it had nothing to do with the Fukushima I Nuclear Power Plant accident.
According to Setagaya-ku and the Ministry of Education and Science, the bottles look to be made of glass, about 7 centimeters long and 5 to 6 centimeters in diameter. There are several tens of bottles in a wooden box. The content of the bottles are powdery.
The radiation was so high that the survey meter that they brought couldn't measure as it went overscale. No one is currently living in that house.
Whether people believe the government is another matter. Some simply do not. I am rather shocked to see the Ministry of Education, of all people, responding so quickly, even releasing the photo above so quickly. That ministry is the last one I'd expect to respond to ordinary people's concern. How ironic.
(Oh wait... The radiation near the surface of the side walk (5 centimeters off) was rather high at 1.34 microsievert/hour, according to Mainichi Shinbun (10/13/2011). Radium in the bottle under the house cannot explain that, can it??)
In the meantime, more radiation "hot spots" (often several microsieverts/hour) have been discovered in the Tokyo metropolitan area. More later.