The 'WASHER FLUID LOW' message or illumination of the indicator is triggered by the rear washer fluid reservoir level sensor.
The rear washer fluid reservoir transfer pump will transfer the washer fluid when it receives a signal from the front washer fluid reservoir level sensor (i.e. when the front washer fluid level reduces to approx 1 litre) and continues until a signal is received confirming the front washer fluid level is full. This is why it’s helpful if a fault is identified with the front sensor (which may or may not be the case here).
As I mentioned earlier, when a 'WASHER FLUID LOW' message appears in the Instrument Cluster's message centre, the first display of this message, or illumination of the indicator, is accompanied by a 'chime' sound to alert the driver to the low fluid level.......I’ve asked the OP if he’s noticed this at all?
Chances are that it's the rear washer fluid reservoir 'Non return valve' that's causing the issue but before removing the rear bumper cover to get at it, it could be worth having a diagnostic check to see if this picks up a fault with the front washer fluid reservoir level sensor or anything else?