Nothing is perfect. So are the fan missions.
Don't get frustrated about lower ratings too easily.
Feel free to be frustrated when you cannot find this specific item hidden in the least obvious place.
Rating will reflect not only analysis but also experience (or emotions).
FM authors are not professional (in most cases) and such imperfections are natural thing to happen.
Hi there!
Assuming you are at Calendra's house, you should have "Lady Calendra's Key" with you. This key will open a floor door to the basement.
See the supreme ghosting video (at time 1:04:12 )
https://www.thiefguild.com/videos/18856/lets-supreme-ghost-thief-calendras-cistern-segmented-run-no-commentary
Cheers
Konrad