Dead Battery and 'beep' 'beep' 'beep'

I went to start my '99 K1500 today after work and received a surprise 'click' 'click' 'click' when I turned the key. I looked at the volt meter
and it was around 11 volts. So I made a quick analyses, battery is dead. Kinda weird as it's not acted up at all, just died while I was at work, battery is the original so I guess it did OK.
So after a jump start the volt meter jumps up to 14 volts so I motor to AutoZone as I do like their Duralast batteries, buy a battery and take it home to install it. Kept the truck running all the while of course.
Ten minutes after getting home, new battery is installed. The weirdest thing is now when I shut the truck off I here a slight 'beep' 'beep' 'beep' coming from the inside of the dash. I've never heard this sound in my truck or any other vehicle that I've owned. This is the first time since the truck was bought that the battery was disconnected. Any ideas what that 'beep' 'beep' 'beep' might be?
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