From time to time my phone with go thorough a phase of sending out a short alert tone and I don't know what it means. It's 3 or 4 quick descending notes. It's different from the low battery one. It appears to be completely random. No apps in use or running in the background, no notifications arrive, the phone can be fully charged, charging or low batt. I'll get this for a few hours then not hear it for several weeks. I can't find the tone in the notifications settings, the tone I use is Tethys and this one I hear isn't available to select.

Any ideas?