Hi Dan,

The battery should measure 12.6 V with engine off.
with the engine running you see the alternator and voltage regulator.
around 14 volts.
14.0 is good higher and you start blowing lights faster.

Check your battery out, how old is it. Many parts stores will check it for you.

It's possible that something decided to draw current when the car is off.
Maybe the radio at the Submarine Races (Lovers Lane) for hours? :-)
Things such as a trunk light or glovebox light can discharge the battery over time.
You can check by measuring current flow at your battery with everything off,
and the car locked so nobody can run the windows and blow out your ammeter fuse.
(Ask me how I know this)
You should find some fluctuating current to run stuff like your clock and
some ECU functions. Expect to see around 20-25 DC milliamps for a normal car.
If it's way higher, start pulling fuses to locate the culprit.
Pull the courtesy lights fuse/s first.

Cheers, Bill