But how could the curtains flutter like that with the window closed?

The curtains in front of the open bedroom window didn't flutter or move.

She looked up to see the curtains fluttering like her lungs.

The curtains over the living room window were fluttering, and he glanced up.

The window was still open, curtains fluttering in the breeze.

The thin white curtain, pulled back, fluttered in the cool air.

The curtains were fluttering like the wings of a bird.

He opens the door and finds the curtains fluttering in the wind.

The curtain under the sink, where I'd hidden as a child, fluttered.

After a minute the curtain fluttered, then he heard the dead bolt click open.