Art of flight, I can see what Arrab is getting at. Supposing something (e.g. water retaining bacteria - not unknown in fuel tanks) caused the capacitance gauge to indicate a higher reading than it should, and also delayed the drying out of the heated wire for the warning gong, the first thing warning the pilot might get is the gong, albeit later than it should be.
The capacitance probes were destroyed in the impact, so they couldn't easily be tested, and as the heated wire sensor would dry out over time if still powered up, the evidence would have been inconclusive by the time the accident investigators reached it. Eurocopter said that some faulty probes on other EC135s 'worked after cleaning', so one would hope the AAIB have been checking for traces of whatever was cleaned off the faulty ones, in the Glasgow helicopter's tanks.