Discovered two days ago (likely due to its small size). Reports BBC News:
The asteroid, estimated to be about 11 m (36 ft) in diameter, was first detected on Wednesday. At its closest, the space rock — named 2012 BX34 — passed within about 60,000 km of Earth — less than a fifth of the distance to the Moon. Astronomers stressed that there had been no cause for concern. “It’s one of the closest approaches recorded,” said Gareth Williams, associate director of the US-based Minor Planet Center.
“It makes it in to the top 20 closest approaches, but it’s sufficiently far away …” he told the BBC. The asteroid’s path made it the closest space-rock to pass by the Earth since object 2011 MD in June 2011.
Here’s more from Space.com: