Pressure Plates/Sound Tiles
I'm not sure if anything like this exists. I'm thinking of possibly an object(custom floor tile/rug/mat) that automatically plays a sound when sims walk over them. A trumpet blast or alarm when visitors are spamming the nursery or even a mat that announced when visitors were going into private areas would alert me to their nefarious wanderings.
Kidney stones are never round pebbles... they are more like jagged stalactites.