Measure from the voltage regulator output + to the battery - before you do anything else (engine running).

If you're still getting just 12.X volts, then the previous advice is sound.

If you're getting 13.5+ volts, then you have voltage losses between the battery and ignition switch, which is likely to be something like corrosion at the fuses or a poor connection.

Have you been worried about the car to the point where the gauge panel was meant as a diagnostic, or was it a way to give yourself another perspective into the car's workings?

If the former; then your alarm is not new, the gauge is helping you diagnose what you already suspect.
If the latter, then you may be giving yourself heartburn when there is no real problem. Have a cup of coffee and a deep breath before you pull out your VOM again. You're not likely on the brink of automotive disaster, you've just learned something new about an old friend...and you have to decide how you feel about that.