Suppose the probes and the sensors had bacterial growth retaining water on them, this would cause the fuel probes to misread and the red lights to not come on either.
When I enquired in this thread about the sensing mode of the red light sensor in the supply tank which activates the red light for 'very low fuel', what was described was something along the lines of a mass flow (MAF) sensor in a car. I.e. Its resistance changes with temperature. In that case, if there was a build up of bacteria sludge on tank components, then this sensor could just as easily be affected, also causing either no or a slow response from it.
Page 44 of the UH-72 Lakota flight manual, which can be read online, gives a good description. Yes, I know its not an EC-135, but the principle is likely to be much the same.
UH-72 Lakota Helicopter Flight Manual - Department Of Defense - Google Books