Can the alarm spell be detected by mundane means such as a normal perception check?

Question

The Alarm spell in the PHB reads as follows:

You set an alarm against unwanted intrusion. Choose a door, a window, or an area within range that is no larger than a 20-foot cube. Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won't set off the alarm. You also choose whether the alarm is mental or audible. A mental alarm alerts you with a ping in your mind if you are within 1 mile of the warded area. This ping awakens you if you are sleeping. An audible alarm produces the sound of a hand bell for 10 seconds within 60 feet

Are there any examples in officially published modules or rulings, errata, etc. that indicate whether the alarm spell is visible via a Perception check vs. the spell's DC, or whether the spell is invisible and must be detected through a spell such as Detect Magic or Find Traps?

Answer 1

Skill checks are of no use, otherwise the description would have specified it.

By description, Alarm does not leave any visible or invisible sign on the window or the door it has been cast on. If it required a Perception check (or a similar one), the description would have stated so, as it happens for example for Spike Growth (emphasis mine):

[...] Any creature that can't see the area at the time the spell is cast must make a Wisdom (Perception) check against your spell save DC to recognize the terrain as hazardous before entering it.

Further examples are

• Glyph of Warding (emphasis mine):

[...] The glyph is nearly invisible and requires a successful Intelligence (Investigation) check against your spell save DC to be found.

• Snare (emphasis mine):

[...] This trap is nearly invisible, requiring a successful Intelligence (Investigation) check against your spell save DC to be discerned.