It means she literally has a habit of doing that, but this behaviour is in turn supposed to suggest a certain sort of character. Exactly what sort of character is to some extent context-dependent and up to the reader to imagine, but I guess it may be anything from observant, to inquisitive, to knowing, to disapproving, and probably some other things mixed in too. In this case the author obviously thinks it's suggestive of "a night-shift detective who has seen it all".
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