The two main classes of black holes pose no threat to distant bystanders. Small ones , of a few solar masses simply sit in the middle of their former solar systems, lit up like lighthouses by infalling matter , and giving fair warning to stay clear.
Giant black holes in galactic centers,like this, the quasar Abel-3, though capable of stellar genocide,devouring or gravitationally disrupting any stars that spiral in, are too deep in their own massive gravity wells to be released by anything short of an inter-galactic core collision .
But now astronomers have circumstantial evidence for the existence of a third class that weigh in at a few hundred to a few thousand suns. They might form within dense, spherical swarms of hundreds of thousands of stars known as globular clusters, although no one is sure how.
General relativity dictates that the merger of two black holes produces a burst of gravitational waves, expanding ripples in spacetime itself. If the merger is eccentric, or if the two black holes spin at different rates or in different directions, gravity waves emerge asymmetrically,and can act like a rocket on the product of the black hole merger flinging it out of the cluster with a velocity of thousands of kilometers per second.
At this weeks meeting of the American Astronomical Society in Austin Texas Kelly Holley-Bockelmann of Vanderbilt University reported that, If intermediate-mass black holes do exist they will be prone to merger, triggering the sort of gravity wave cusps that could spit them out of the clusters and send them tearing through space at several per cent of the speed of light
Isn't astrophysics wonderful- the worst thing that can happen to the solar system just keeps getting better and better. The good news is that even if hundreds are being generated annually, the odds against our getting clobbered by one of these double barreled monstrosities are a quadrillion to one.