Compared to many other countries, the crime rate in Japan is relatively low - a cat being stabbed makes the top evening news for example (yep - cats get stabbed from time to time).
This yellow sign is telling people that they should lock their doors when they leave the house - many people dont lock the front door because it is so safe.
But then again, it depends on where you live. I have always lived in the more better off neighborhoods whilst in Japan (because I used to live in a dump called HACKNEY back in London). There are indeed areas where for some reason, crime rates concentrate in areas where there are many non-japanese living. Unfortunately - many foreigners come to Japan to do bad things which is why many Japanese have a bad impression of foreigners altogether :-(
