Post box appears in the middle of the Thames

Sarah Coles
Postbox on the Thames
Postbox on the Thames

Villagers in the Berkshire village of Sonning-on-Thames have been baffled by a red letter box, which appeared overnight. Bizarrely, it's on one of the legs of this bridge in the middle of the river Thames, which is only accessibly by boat.

So how did it get there, and is this the weirdest thing to happen to a letter box?