My 1.6 petrol bhp156 warned of having low oil level so I topped up with 1 litre but didn't clear the message??!
Park it on a basically flat surface (as in, not a steep driveway) and let it sit so you give it time for a good deal of oil to run back down into the sump.
Then after lets say an hour, use the dipstick to check and you'll get a proper reading. If the dipstick shows the oil is at max, then you can ignore the alert.
As someone else said, alert messages can take time to clear but I would say, if its been a day or two and the message hasn't cleared despite you confirming the oil level is good, then you probably would have a faulty sensor.