Im not sure about that i have had the warning within 5 minutes of topping it up beforeNot sure we are talking about the same thing?
A permanent warning until opening the flap makes sense of course, however the way the A2 appears to work is that If for whatever reason you get the warning, open the flap, then don't fill up (or add way too little), the warning is still going to be disabled for the next 100km, regardless of whether it should be there or not.
I seriously hope that it does not "lock out" all new warnings (yellow or red) though have not been unlucky enough yet to get to test that. (Example: You've filled up windscreen washer fluid, then 20 miles later, the Oil MIN warning is triggered by the sensor. Does it show up because it's a new warning or does it not because opening the flap disables all warnings for 100km?)