Shababniks know their kehillos intimately. An article in the English Hamodia 2 weeks ago told how they know when familes are away at simchos by looking at shul notice boards announcing brisos, weddings etc. They know that most Haredim won't call the police on shabbos, even though it is Halachically mandated to do so. Although cars were absolutely banned in Bnei Brak on shabbos, so great is the problem that beefy Russian goyim have now been hired to patrol Bnei Brak in cars on shabbos.
Another possibility is that the thieves were part of the growing number, in the hundreds of thousands, of Halachic Jews, who have Jewish mothers but Arab Muslim fathers. Almost without exception they take after their paternal evil character and religion, and fanatically hate Jews and Judaism. Alll they have to do is put on a kippa and tzitzis, and they can walk around masquerading in Bnei Brak as real Sefardi Shasniks, but with a detectable un-Jewish glint of killer in their eyes, detectable to the discerning!
With all the internal fighting, rivalries, and recent revelations of arrests, jailings, sex crimes, laundering and financial crimes, there is a growing feeling of Haredi self-hate, and Haredim, although still better than 97% of humanity, do not look like to the world or to eachother as "the elect of mankind" any more. Haredi society could quickly disintegrate in the next few years, as has happened again and again and again throught Jewish history.