It would be very unusual for the overflow to siphon out water from a tank that's less than half full, same with thermal expansion. City water fill valve or pump check valve leaking and letting the tank fill up is more reasonable, though the gauge would presumably indicate a more full tank and attempting to fill would of course immediately result in a stream out the overflow if it is indeed full.
If the gauge reads in percentages, rather than just having thee or four lights, then it presumably is a capacitive gauge and should be pretty reliable and accurate unless the sensor got detached from the outside of the tank. Even the sensor probe ones tend to be reasonably reliable in the fresh water tank since there isn't (supposed to be) muck to foul the sensors and cause false readings.
Is it possible the drip is coming from something other than the overflow? Might it be condensation on the tank, or a slight drip from a low point drain valve or tank drain valve that isn't fully closed or is leaking, or something of that sort?
