The sign on the museum door says in Estonian, “There is nothing here,” a contemporary joke about a meandering set of rooms atop what had once been the country’s most modern hotel. These were offices of the KGB, the “Committee for State Security”: the Soviet agency that spied on foreigners and Soviet citizens with equal suspicion, distrust and disdain. The rooms are now a museum in the Viru Hotel, which still operates in the center of Tallinn, the nation’s charming and historic capital.