You sure you just didn't have your truck go into deep sleep or throw some other error and have the sound play from the in-dash speaker instead of the sound system?
That s probably it. How long does the truck need to sit before the Deep Sleep mode comes on?
It really depends. In theory, the truck should keep the 12V battery charged even when it's off, but the Lightning has been plagued with low 12V battery voltage issues. So I couldn't tell you exactly how long, but I'd say if it's happening after a shirt time, it's usually related to charging somehow. If my truck isn't plugged in, it doesn't usually have a problem for several days if it does at all.
I agree, however this anomaly is different and I think a previous comment nailed it down. Sometimes the chimes come out of the in-dash speaker, other times it comes out of the Sync system. Something related to whether the truck put itself into deep sleep mode.
The chime should be the lower, gentler one. I have noticed the shriller chime after my truck has gone into a "deep sleep", not sure if they still call it that. Where it basically doesn't/can't engage the sound system prior to needing the chime to play, so it uses some older fashioned backup sound device behind the dashboard instead. This is usually accompanied by a "Electrical System Drain" warning where my LVB was low, and why it went into a deeper state of "off"
