Right, so this is a weird one.
LG G4 on Rogers running Android 6.0
I'm randomly getting a loud notification sound late at night that wakes me up, but has no notification that comes with it (so I can't tell what app it's from).
Here's the weird stuff:
Sound, but no notification.
Happens exactly on the hour, but not a consistent hour. I've had it happen at midnight, 1am, and 2am. I can't say with 100% certainty that it doesn't happen during the day, as my phone is on vibrate at work, but it hasn't happened on the weekends.
Not every day. The last two times were last night (friday), and on monday?
Plays the default notification sound.
I have a Tasker profile set up that mutes my notification volumes at night. The only thing that has volume is the ringer. Which means that despite playing the default notification sound, the sound is coming through as either a ringtone or an alarm??
I installed and monitored a Notification History app When it went off last night, I checked it immediately. But according to the app (which seems to monitor damn near everything), there hadn't been any new notifications in over an hour! (Which is true, with respect to every other notification.)
I've also double checked my email app to confirm no new emails arrived that it wouldn't show notifications for (like promotions). Nothing.
Because I've seen it as an answer on other mystery sound threads, I'll confirm that NFC is indeed off.