For those asking what the air defense was doing, here is the destruction of one of the drones in Kazan. 

Drones like these are small, made of radar absorbent materials, fly at low altitudes and do not run hot enough to trip IR sensors. They are very difficult to detect, and with the help of NATO reconnaissance and intelligence, they can exploit gaps in the Russian air defense networks. 

Once they pass the front lines, the density of radars drops off radically, as does the number of available weapons to respond to any threats. Air defense simply cannot be everywhere in a country the size of Russia. 

None of this is to say Russia's air defense is perfect, or that someone didn't fail here, but these sorts of attacks are very difficult to prevent. 

Adding: ❗️Among recent (yesterdary?) other targets, the Russian military recently hit a CIA station in Kiev - Alexander Stepanov, program director at the Political Sciences Academy

“Available reports indicate that, apart from energy facilities, military components of NATO’s intelligence proxies were struck, including a base of special group `Alpha’ of Ukraine’s Security Service and a CIA station at Holiday Inn,” the military expert specified.