This is the 13627th most frequent Russian word.
незаметно
Unnoticeably.
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.
Время летит незаметно, когда ты увлечен.
Time flies unnoticed when you are engaged.
In this usage, 'незаметно' suggests that the action was done subtly or without drawing attention.