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I just did this quest today. A piece of paper should have spawned on the floor. Try disabling all addons and exit the game. Once the game completely shuts down, try to reopen the game.
As a quick work around if this is stopping your progression, although painful, after all other options are exhausted, abandon the quest and redo the last few steps to get up to that point. Usually, that would get it to work.
Same thing happened to me. No note on the floor, no note anywhere. I had to abandon the quest and do the whole thing over again, not happy. Slightly annoying, but it worked the second time. Just blow the whistle before going upstairs and then follow her.
Got the note, but when I get to the prompt from the NPC (Claudia) to USE the whistle, there is no option to use it - ANYWHERE. And yes, I have tried logging out and back in. No luck.