The epigraph gives a clue. (That's the quote at the beginning of the chapter).

The first two lines and the attribution of the epigraph can be ignored for now.

Don't look underneath the door. There's nothing important there.

This points out what's important. Does the top part of the symbol resemble anything in the chapter?

This also helps point out what's important. Does it resemble anything in the chapter?

Focus on the separators between the paragraphs.

They are not all the same. Take note of all of them and what's different.

Note the number of squares in each one. Remember the epigraph.

The number of squares in each separator matches something directly beneath it.