After completing the Wraith mark hunt, if you go into the residence
in Lowtown you can find the key item [DUSTY LETTER] on the table
near the Seeq. Reading it, you can find the clue:

East -> Southeast -> East -> Southwest -> Southeast

Now, this may seem like directions for navigating the Garamsythe
waterway (as the letter talks about in detail, but not in the key
item description), but in reality, it's the method for operating
the switches in the "Central Waterway Control" area. Go down there
and face south, so the switches look like (from north facing):


So, with all the switches starting in the "off" position (not lit
up), flip the switches. If you don't like directions, do it by the
name of the switch:

1) Close No. 11 Control | Once this simple task is completed,
2) Close No. 04 Control | you should hear a "tinkle" noise, but
3) Open No. 11 Control | will receive NO notification that
4) Close No. 03 Control | anything has happened. Head to the
5) Open No. 04 Control | south of the area near the entrance
--------------------------' to the No. 1 Cloaca and you will find
a "Shiny Object" on the ground. Claim it for your own to see it is
a [DULL FRAGMENT] key item.

So turn everything off then start at step 3.