There aren't any dead giveaways as to who's responsible, but the drone landing comes right as a court approved the restart of a nuclear power station. Abe has been keen to revive nuclear power in the years since the earthquake and tsunami that ruined the Fukushima nuclear reactor, but he still faces a lot of opposition -- this may well be an attention-grabbing protest. Whoever's responsible, the touchdown could have Japan thinking about tighter restrictions on drone flights around government offices.

Update 4/25: The drone's pilot, Yasuo Yamamoto, has turned himself in. Sure enough, he was protesting the government's pro-nuclear energy policy.

[Image credit: Jiji Press/AFP/Getty Images]