most likely Jim, no obvious leak- recalls every 3 months for the next year till the heater goes pop and they give up.
E15 can also be a no or slow fill fault if there is no water in the base.
If its a full metal Tank then it’s most likely to be the sump seal. They leak but normally only on the higher temp washes. There is an additional seal kit that can be fitted – in situ – or it requires a new improved sump.