Intermittent Fuel Guage
Same thing happened to my wife's 1995 C280 several years ago. It usually happened when she fueled the car with BP gasoline. Not sure if it was something in the fuel or possibly the fueling process at the station. When she fueled up, the gas gauge would read zero and then after a period of time the low fuel indicator would light. Sometimes the gauge "drifted" a little up and down turning on and off the low fuel light. Sometimes when the gauge was reading zero you could turn the engine off and if you turned the key to the "on" position several times the fuel gauge would read accurately (full) again. This would seem to indicate some type of contamination on the fuel sending unit.
Bottom line is we stopped using BP fuel and this has not occurred again for the last three years. Try using a different fuel or gas station before you go to the expense of replacing the fuel sending unit. You may need to run several tanks of gas to clean things out.