The text for the Alarm spell says that creatures aware of the spell can avoid triggering the spell with a successful Stealth check. But are creatures supposed to generally be able to be aware that there is an Alarm spell? Obviously, if they witness the spell being cast, they're aware of it. And after the spell has been cast but while it's still in effect, a creature using Detect Magic can become aware of it that way. And granted, using Detect Magic is considered such a common exploratory tactic that it has its own entry.
But are those two circumstances (saw the spell being cast or Detect Magicked it after) the only two circumstances where a creature can be aware of an Alarm spell or is it supposed to have some perceivable element beyond those two circumstances?
Alarm is often used as a plot device in scenarios where the author wants to make sure the big bad has time to prepare. I guess that is why Alarm is not simply a magical trap that can be disarmed like any other magical trap. I feel this is bad game design, a group that is good at infiltration tactics has spent resources to be that way - it is extremely unfair that such a group be denied the surprise they worked for and forced into the same combat a noisy group would face.
In PF2, detect magic would be the way to spot Alarm. A rogue that lacks that ability has no recourse - but might be using Stealth as an exploration tactic anyway and so get a chance to sneak past.