Hazards unclear

[playtester]

I recently dove into PF2 rules and I stumbled upon a few things that I don't understand regarding hazards. On page 342 of the rulebook it states:

Stealth DC (p.342 Playtest Rulebook)

This is the hazard’s Stealth DC, which it uses to avoid
being detected. A complex hazard instead lists its Stealth
modifier for rolling initiative, followed by a DC if there’s
a chance someone might detect it.

1. So does that mean that a complex hazards such as

Quicksand (p.18 Playtest Bestiary),
The Iron Maiden Trap (p.18),
The Wheel of Misery (p.19)
or the Lava Flame Tube (p.19)

that have no Stealth DC are undetectable?

2. Complex hazards roll initiative with Stealth, but why does it for instance say Stealth +11 (trained) on the Quicksand hazard, or Stealth +12 (expert) on the Iron Maiden Trap?

Why is this information (trained, expert, legendary) relevant, if the Stealth modifier is only used for initiative and NOT for detecting the trap?

[Trastone]

Only people with that level of proficiency can roll. Everyone else auto fails.

[Franz Lunzer]

Expanding a bit on Trastones correct answer:

Yes, the rules aren't exactly clear, but Mark Seifter remarked that you can detect complex hazards.

The Quicksand can be detected by someone who is trained in Perception rolling a 21 (Stealth +11 => DC 21).