In the latest addition to a growing list of cases that look unlikely ever to be resolved, a Chechen dissident, Umar Israilov, was gunned down in broad daylight in the Austrian capital on January 23.
Others cases include the 1989 killing of Abdul Rahman Ghassemlou, the head of a Kurdish opposition group in Iranian Kurdistan; and the attempted kidnapping in October 2008 of Kazakhstan's former intelligence chief Alnur Musayev. Both were living in exile in Austria.
"Austria is a textbook case for this sort of operation that always remains unresolved. As soon as there is any sort of political link, the authorities start acting very strangely," said journalist Kid Moechel, an author of a book on the subject.
For Peter Pilz, defence expert for the opposition Green party, "some regimes such as Russia and Iran enjoy a freedom to do as they please in Vienna that they would never enjoy elsewhere."
"Quite simply, the Austrian authorities don't want to jeopardise their country's economic interests," the parliamentarian told AFP.
He accused the Interior Ministry of trying to "cover up" the murder of Israilov, who had repeatedly asked for special police protection before he was gunned down while out grocery shopping last month.
Vienna, whose geographical position makes it a point of contact between East and West and North and South, has one of the highest densities of spies in the world, experts say.
It is home to international groups such as the International Atomic Energy Agency (IAEA), the Organization of Petroleum Exporting Countries (OPEC) and the Organization for Security and Co-operation in Europe (OSCE).
In all, at least 17,000 diplomats are based in Vienna, equivalent to around one percent of the city's population, according to official figures obtained by AFP.
"Around half of these have links to the secret services," said Beer.
Politician Pilz asserted that Vienna is also "a hub where it's very easy to buy arms or hide or launder money."
However, the advent in recent years of hundreds of thousands of refugees in Austria, including around 20,000 Chechens, is providing new impetus for secret service activity.
"Every embassy watches its nationals very closely, particularly members of minorities," said Moechel.
Beer said: "Embassies such as the Russian or the Chinese embassies are growing rapidly."
According to some estimates, Russia has at least 500 secret service agents in Vienna, many of whom monitor Chechen exiles.
Austria has admitted to working with Russia's FSB intelligence service -- the former KGB -- in the fight against terrorism.
And, according to the three experts, Vienna also collaborates with the secret services of a number of other countries, sometimes to the chagrin of the United States.
Occasionally, officials overstep the mark: the interior ministry confirmed this week that it had suspended two police officers who had been trying to find out the whereabouts of Rakhat Aliyev, the former son-in-law of Kazakh President Nursultan Nazarbayev.
Aliyev, the former Kazakh ambassador to Austria, has been convicted in his home country of kidnapping and murder.
But he has always maintained his innocence and Vienna refused to extradite him in August 2007 on the grounds that he would not be given a fair trial at home. Officially at least, his current whereabouts are unknown.
In its annual report, the interior ministry acknowledged that "Austria will remain a field of operation for foreign services, as is seen in the very large number of agents." But the ministry did not provide any concrete figure.