This is the 13627th most frequent Russian word.
незаметно
Unnoticeably.
In this usage, 'незаметно' suggests that the action was done subtly or without drawing attention.
Она незаметно поправила свои волосы перед зеркалом.
She discreetly adjusted her hair in front of the mirror.
Here, 'незаметно' conveys that an action was performed in a way that wasn't noticed by others.
Он незаметно покинул комнату.
He left the room unnoticed.
In this sentence, 'незаметно' describes how time passes quickly without being felt or noticed.