So, I checked to see if it was vibrations she was feeling (instead of hearing). First, I stood in the tub and did it...nothing. So, then I had Hubby stand in the tub....nothing. So, then I had Miss Kat stand out in the hallway, on different floor material, and tried again. Yep, she still heard it. Finally, I had Hubby stand in the hall, on a pillow, holding Miss Kat, with her back to me. Dropped the lid three more times, she heard it each time.
Guess she IS hearing it. Maybe I'll get the audiologist to take her in the booth and see what they find.