Gas Gauge Not Reading Properly

I have a 2003 explorer 4.0 and i just filled up this afternoon drove 1 mile ate supper and got groceries. I came out now the gas gauge is reading only 1/2 tank. is there anything short of replacing sending unit that could fix this. I guess someone could have stolen some gas but damn, in broad daylight in a crowded food lion parking lot. This is the first time this has happened since I have owned the truck. BTW it has 106000 miles on it.