I dont think they own a single bitcoin. If a central bank buys something you would see obvious clues about that.
A central bank doesnt buy 1000$ worth of bitcoins, they buy them in billion $ quantities.
If they would be serious then the bitcoin market cap now would be like 1 trillion $ of which 98% would be from central banks.
If they are not serious , then they dont buy a single one because they got other things to worry about.
Central banks/governments do not have to buy bitcoins.
Seized bitcoins (like Silk Road bitcoins) are included in their kitty too.
In the future, if governments allow payments for services through bitcoins (there is already some talk regarding this), their holdings will increase.
They already auctioned those coins, so i dont think they hold them. Besides that was a law enforcement agency holding the coins and not the treasury.
There is a difference between coins held as evidence in a crime, or coins held in a treasury account which then can be used for spending or who knows what.