[Warning Light] Adblu again

[T33CNO]

Warning light does not exist on my 2015 model but I get a really loud anoying bleeping when the adblu needs attention. There is no info on how to shut it up other than to fill the tank. This has happened twice now since buying it second hand. The main issue it that both times it only took 2 litres at most to top the tank up. I thought this was a 17 litre tank? so there should not be a warning.

Any help appreciated.