Think in the shadow. What does it mean? Is it an idiom?

Question

Ian sits in the dark, thinking in the shadows.

I've across the phrase above in a novel.

I don't understand the meaning.

Does "shadows" symply mean "shadows"?

Or "thinking in the shadows" is an idiom? Could you please explain it to me?

The fuller text:

Beverly thought she caught an odd look on Ian’s face in the shadows, when they brought Bradley in. Something cold in his eyes that she hadn’t seen before. It gave her a chill. She’s not sure—the expression was so fleeting. She might have imagined it. [...] His wife[Beverly] appears silently at his side, kneeling down by his chair, making him jump. It’s almost as if she’s read his mind. “Henry,” she whispers, her voice so low that he has to lean his ear down next to her lips to hear what she wants to tell him. He can smell her breath. He wonders if she can possibly know what he’s thinking. “I think I know who the killer is.” He raises his head and looks at her frightened eyes, gleaming in the dark. Ian doesn’t like the way Beverly’s been looking at him. She’s gone over to her husband and is leaning close to him, whispering something in his ear. That’s interesting, seeing as usually she’s stayed a good distance away from her husband lately. He wonders what she’s saying. Maybe something about him. Ian sits in the dark, thinking in the shadows.

Answer 1

The sentence can be simplified to:

Ian sits in the dark, thinking.

The addition of in the shadows is simply to set the mood poetically or descriptively.

It could also have been written as:

Ian sits among the shadows, thinking.

Or, in different descriptive situations:

Ian sits in the dark, thinking about his plight.
Ian sits in the dark, writing in chalk on the floor.

In other words, thinking in the shadows is not an idiom or a set expression. It's just being descriptive.

More than that, it's actually a stylistic reference back to the opening sentence of the paragraph:

Beverly thought she caught an odd look on Ian’s face in the shadows . . . Ian sits in the dark, thinking in the shadows.