If it helps, I had a warning for low coolant level and so bought a bottle of Volkswagon G13 coolant ........

 

It is still in my cupboard unopened, as before it arrived I realised just removing and replacing the cap had re-set the warning light and it's been fine ever since.

@Tilt

Since the cap has no direct bearing on the coolant level electrical sensing, I think it's safe to assume you must have disturbed the sensor electrical connector and remade what was a bad connection.

Unless there was pressurised gas above the coolant, depressing its level. Take cap off, release excess gas, level rises, warning no longer registers?

@Pete_Ex-Wino

I would imagine the pressure relief cap would have to be faulty and/or the coolant level borderline low to begin with... anyway it worked for Tilt.