This happened to me after I did a oil change; the warning signal came on about 500miles later. I knew I topped it off. So , I wanted to see how sensitive the sensor was so I evacuated (sucked out) 1oz. at a time and drove the car. After 4oz's were withdrawn , the warning no longer appeared.
I like ChipB's recommendation: buy a dip stick (that was the First item I purchased for the car)! This way you can diagnose your sensor and compare the readings; and you will have peace of mind that the oil level is "in range".
My Theory: Also, beware that a "sticky" or dripping fuel injector could "Add" to the volume of oil in the sump! (I have had this experience with another make and model of car) ; another reason to have VCDS and check for fault codes. After several onces of fuel have leaked down, it will show on the dip stick and it sounds like that sensor is very sensitive. Keep an eye on the stick especially if you haven't had a oil change recently!
I think thats more plauseable than "wiping off the filler cap"

That is exactly what a dealer would say - that myth probably started in the sales dept. (and they are still laughing about it)!