I dunno about now, but these small explosions used to be fairly common in Moscow.  I mean, not every day or anything, but not infrequent either.  They were usually attributed to organized crime, not terrorism.  (Or to gas explosions, also not-unheard-of....)  And whatever the cause, at least back then (in the pre-Putin days) it was standard practice for the authorities to downplay it or lie about it.  I don't know if I see anything new here, although it would be interesting to know what exactly got blown up and how big the blast(s) were.