Particulate filter fluid top up.

[GiveMeABreak]

The additive check on your specific vehicle would be done at 60k miles, then every 20k miles and then topped up if necessary, but most will wait for the warning message to come on first advising of low fluid level if it has insufficient fluid remaining from the point of the alert to the service due based on your maintenance indicator. If it thinks there is insufficient additive to last until then, it will flag the warning up. So I suspect it is about due for a top up and reset any time.

[NotAnInterestingName]

Thanks, will leave it another year and see what happens.

Does it actually sense the level, or does it just rely on being told when it's been filled and counting down the miles or tank-fills?

[GiveMeABreak]

There is no physical measurement of the additive tank.

It is estimated and calculated based on the original amount filled at factory and the number of metered doses injected into the fuel tank (whenever a certain quantity of fuel is added).