This is the 573rd most frequent Finnish word.
hälytyksen
The Finnish word "hälytyksen" means "of the alarm" (possessive/genitive form).
In this sentence, 'hälytyksen' refers to the alarm sound that was heard.
Hän kuuli hälytyksen ja juoksi ulos talosta.
He heard the alarm and ran out of the house.
Here, 'hälytyksen' signifies the event of an alarm being issued.
Pelastajat saapuivat heti hälytyksen jälkeen.
The rescuers arrived immediately after the alarm.
In this usage, 'hälytyksen' indicates possession, tying the object (alarm) to the action (resetting or rearming).