The Fiction

She's pretty casual. Doesn't initiate much, keeps things light, hasn't said anything that sounds like feelings. You take that as the read.

Then things that don't fit start accumulating. She remembered something you mentioned once and let drop. She mentioned another guy, and the timing was too deliberate to be accidental. You find out she'd been sitting on something she thought might bother you, waiting for the right moment to say it.

Indifferent people don't do any of that.

The game

She has two options: show where she actually is, or perform something else. Performing casualness costs almost nothing and preserves her position. You don't know she's invested, so you can't use it. The leverage stays even.

The cost of performing is that it requires active work. She has to decide what to share, track how things land, time what she says. Genuine indifference doesn't involve any of that. It just acts.

The tells are behavioral: remembering things, managing information, sending a signal when she wants a response. These require effort, and effort contradicts the casual front. She can't be genuinely uninterested and carefully managing your perception at the same time.

Dominated strategies

Taking the performance at face value. She seems casual, so you act casual, so you never find out what was actually there. The fiction runs on your cooperation: she doesn't have to drop it if you never test it.

Reading her signals as neutral information rather than moves. When she mentions another guy at a deliberate moment, she's not reporting. She's watching how you respond. Treating it as a fact means responding to the wrong thing entirely.

Genuine indifference doesn't manage anything. It doesn't remember the thing you mentioned once, doesn't time what it says, doesn't sit on something waiting for the right moment. If she's doing any of that, she already told you where she is. You just took the front at face value instead of the effort behind it.