@feld @Moon @a1ba One time a colleague went out and left his Nokia phone (not a smartphone but already with polyphonic ringer) on his table. I took it, reduced the ring volume to minimum, wrapped it in some plastic bags and glued it with the tape under his table. He came back, didn't find his phone and called it.
Then he spent 30 minutes turning the room upside down trying to find where that faintest sound came from. The famous Nokia ringtone turned out to be super hard to pinpoint.