Were they in the room, behind the curtains, or just outside the window?

He has been the man behind the curtain all along.

I take the bed by the wall, behind the curtain.

"They want to go behind the curtain to see the dark side of things."

In other words, pay no attention to the man behind the curtain.

So pay no attention to the man behind the curtain.

How could anything bad happen behind a curtain like that?

He saw the figure pass behind the curtains of the other window.

A small female figure is looking from behind the curtain of a house.

Then he got up, walked behind the curtain and was gone.