Well, since there are no established facts of governments buying Bitcoin, we can either assume that they never did, or that they did it quietly. For the sake of argument, lets agree on the latter.
Would they keep the stuff nicked by law enforcement? Look into mining? Buy through intermediaries? Be open about it or do it on the sly to make sure us scum didn't pump to rinse them? Would they collude with other governments or establish a position in secret before anyone else?
Seized coins - seems very unreliable. The US and some other government auction seized coins from time to time. Although
the story about Bularia's 200,000 coins is pretty interesting.
Mining - needs very cheap electricity to work in the long run, so not all countries can afford it. Also it's hard to hide.
Buy through intermediaries - yeah, OTC buying would be my first choice.
I think authoritarian and corrupt governments have more chances to get into Bitcoin, because it's harder to hide anything in democratic government where parties watch each other and the government changes every 4-5 years. There are rumors that North Korea was behind some exchange hacks and now has a bit of coins.