Passat 2017 B8 Coolant Loss Issues

[ANDY54]

Our 2017 Passat TDI 127,000 km has a mystery coolant loss. On recent inspection coolant tank was empty without any warning symbol displayed on dash. Simple test for cap on tank holding pressure done ie drive until oil temp shows 90 degrees then leave overnight. Next day when cap un-screwed , no sound of pressure release. Oil is not milky and no visible signs of coolant around engine bay. That's not to say I haven't missed something.

Any thoughts ?

[ANDY54]

After working on this problem today I'm answering my own question for anyone who finds themselves with a similar problem.

I fitted a new cap which seems to have kept pressure. But the big find was with the two probes which were encrusted with some sort of chemical buildup. When these were cleaned they gave a warning of very low coolant level on the dash.