What appears to be frosted glass in the right hand pane is in fact tiny pieces of broken glass, still held together in the frame. Nobody had been in the room overnight and it was the inner pane.
On examination the breakage was seen to radiate from a point somewhere behind the grey top on a coat hanger on the clothes airer.
The fact that the glass had held in place and therefore enabled closer scrutiny showed this distinctive pattern at the centre of things:
click to enlarge
This, I now know (thank you Google) shows evidence of a spontaneous break caused by an impurity in the glass at manufacture. The distinctive feature is the two larger pieces in the middle, known in the trade as a butterfly pattern - rather like a pair of butterfly wings with adjacent parallels between the two. Though rare, they are most usually caused by a minute piece of nickle sulphide which can be seen on close inspection. The broken pane is creaking and will almost certainly fall apart without warning soon. Be assured I am going nowhere near it in search of a speck of nickel sulphide! The picture was taken from the outside, through the intact outer pane therefore.
Had the window been a heap of glass fragments on the floor it would be very hard to discover these two tell-tale shapes among millions of pieces of glass.