I have had this lately too. At first I thought it was to do with USB connected (when charging) but it wasn't. I even disabled "Auto Launch - Start App when my phone conencts to my car dock" in Car\Settings (app).
Have tried power down re-boot. No Luck
(NOTE: When you reboot to fix issues be sure to always toggle off "Fast boot" in Settings\Power).
A few times when it happened it launched the 'choose default app' notifcation. One of the options was an 'Alarm Clock' app. I have uninstalled all the apps that it was showing to launch this mystery notification but it still happens!
This is bugging me and I hope more people can contribute to this thread!
I called HTC Support (Australia) today & reported it but my report was overshadowed by the main reason I called them: After using it early this morning, my fully charged phone would not wake up or boot up later on this morning! D E A D...
I fixed it (thankfully) by: (1) Plugging in wall charger (2) Press+Held (the centre of) Vol up/down key (3) Press+Held Power key. Bingo, it booted up. Simply by pressing the power key for over a minute would not work.
This allegedly re-calibrates the battery (courtesy of a
support page on T-Mobile). I hope it was not related!
Summary:
(1) I believe it is USB related.
-- Possibly charger related. (I.E. Either from using a non-stock wall charger
and-or using a non-stock USB cable (or even a USB cable that only supports charging and not data transfer).
(2) Through some condition (once again I think USB related) I think it is sensing something or other and trying to warn with a notification.. but not the system notification just the Ringtone.
(3) It is always my main "Ringtone".
Not my "Notification sound" (i.e. found in Settings\Personalize\Sound).
(4) There is no incoming call.
(5) It happens when phone in standby, no launched apps, nothing electrically connected and in flight mode.
(6) It has not happened on a 'spare' HTC M8 I have (which I received from HTC Australia as a prize at a Telco industry HTC M8 product launch). This phone is not connected to a mobile network, only via Wi-Fi to my DSL service.
(7) It bugs the
hellout of me!