I still haven't got my head around whether AGNs (active galactic nuclei) can go undetected or not... but it's quite possible that a microquasar might sneek up and zap us in 2012:

" Astronomers are keenly interested in SS 433 because it seems to be a miniature, million-times-scaled-down version of the engines that power quasars and other active galactic nuclei. This bizarre system consists of a fairly normal star and an extremely dense object — either a neutron star or a black hole — that orbit each other every 13 days. A stream of gas is spilling from the "normal" star into the dense companions's deep gravitational field, where it swirls into a hot, brilliantly glowing disk. Somehow, much of the material in the disk ends up within two narrow, oppositely directed jets that shoot away from the collapsed object at a quarter the speed of light. "

(jets that could do serious damage to life on our planet...)