The look that stays just long enough to make him second-guess himself

He says something forgettable, maybe about the song playing or who just walked in, and she looks up. Normally she would blink first, smile politely, give the floor its little rescue glance. This time she stays there.
It is barely a second too long. Long enough for his sentence to trail off weirdly. Long enough for him to pick up his phone and pretend there was a notification.
If someone across the room noticed, they would call it eye contact. He knows better, or thinks he does, which is worse. She gave him nothing he can quote later. Just that pause.
The outfit stops having a believable excuse

She still has an excuse ready. It was the first thing she grabbed. She was running late. The dress only looks like that because of the chair, the light, the angle, whatever.
But the excuse starts doing a bad job. The sweater slips and she lets it. The jacket comes off before the drink even arrives. The hem gets smoothed once, then left alone. She is not yanking everything back into the safest version of itself.
That is what he notices. Not some huge change. Just the absence of panic. The mirror was involved before she left, and now everyone is expected to play along like it was accidental.
She stops laughing every time the tension gets obvious

He gives the compliment a little too directly, and the old script would have saved everybody. Quick laugh. Dumb joke. “Shut up.” Maybe a fake complaint about looking tired.
Instead, she lets it land. She looks at her glass for a second and says, “Thank you.” No scramble. No wrapping it in embarrassment so he can pretend he meant nothing by it.
The table keeps moving, but he does not. A plain thank-you can feel more dangerous than flirting when it comes late and calm. It tells him she heard exactly where he aimed it.
The small adjustment happens after he looks, not before

A strap fixed before anyone sees it is just a strap. Hair brushed back before the conversation starts barely registers.
The version he remembers happens after his eyes mess up. She catches the glance. Waits. Then a thumb slips under the necklace chain, or she pulls the sleeve back into place, or smooths the skirt with one hand before looking up again.
Nothing big enough to accuse her of. That is the problem. The order is already stuck in his head: look, pause, adjustment, that almost-smile he will swear was there even if it was not.