Fair warning, you dirty Gaijin.

Fake Police May be Casing Foreigners, Stealing Identities | Japan Probe:

"If you are a foreigner in Japan, you may want to take a good look at the officer the next time you are asked to show your ID, especially if at your own home. I just caught this story at a online blog community called Expats Japan... the poster tells of a suspicious cop who came to her home and asked for personal information. She later called the local police and sure enough, they knew of no such information collecting activities being performed by their own officers. They offered the following advice:

1) the police department usually sends officers out in pairs, not alone, to someone’s door
2) if the police want to collect personal info, they will tell you explicitly what it’s for (and this would be a very rare case). if language is an issue, they would encourage you to call a japanese speaking friend or would just leave and return later with an (insert your language here) speaker
3) police officers will always show clear IDs, and you can copy down their ID numbers for your records
4) police officers who come to your door will always be wearing large badges, also with ID numbers
5) and of course the usual….always keep your door locked, hide your valuables, don’t answer the door to anyone you don't know..."