I read in An unwanted guest by Shari Lapena the sentence below:
The younger man behind the desk does an almost imperceptible double take, but it's there.
What does it mean, but it's there.
The fuller text is here:
...he glances up when he sees her. He stops for a second, stares, and then smiles in an embarrassed way and looks away. She's used to it. She has that effect on men. As if when they see her, they can't believe their eyes for a minute. She can't help that. The younger man behind the desk does an almost imperceptible double take, but it's there. She's used to that, too...