I was collecting echo shards, at least TRYING to find some, get 2 warning and go afk 10 minutes to reduce the warning level, and then I checked the recipe for the next bookshelf (yes, I was crafting bookshelfs for enchantments) and saw "warden tetril", which oddy looked like the warden's antenna.
I was searching the recipe, but there were none.
I was looking in the chests, but still.
None.
I was confused and since I'm bad at the game, I triggered the sensor a third time, which was the last warning before the warden.
And this is there I realize.
This wasn't a craftable item.
This was a loot.
A mob loot.
The mob loot from the warden.
I had 8 echo shards
Now, I had to trigger the sensor a last time.
And somehow defeat that beast.