ITs not unheard of for the sensors to fail, I've seen threads on the topic. Its a fairly cheap part like in the $30 range, so it might be worth trying to just replace and see.

have you been taking the oil level measurement always on level ground and after the engine has been stopped for at least 5 minutes? The reason being is that the engine can show low on the dipstick but still be at the proper level.

My 4.8i leaked a little and burned a little (bad valve stem seals) but it wasn't significant enough that I had to add more than 1 qt every 5K miles or so. My oil level sensor would show the level gradually falling over time but take about 10 minutes of driving before it would register a change in level.