Intermittent fault code

[monty quick]

'System Fault Code 128 - Water Sensor' has come up 3 times in the last few weeks. I can clear it and it has returned after many miles on one occasion and just after starting on another occasion. I had someone look at the logs and they say it has not been logged.
Any ideas of what the code refers to and if I should be worried about it?
Thanks in advance.