Code E15 on Bosch Exxcel ?

Although turned off, and nearly empty, the machine started to show E15 and could not be turned off except at the mains. As suggested in manual I have checked water supply and filter. When turned on again the machine "churns" underneath and then re-shows E15. There is no obvious leakage of water. I have tried turning it on it's back and rocking it from side to side radially to see if any water drained out. It produced about 0.5 litres which I would have thought was normal residual water, but I may be wrong and maybe nothing should have appeared The machine's EWR is SMS50E02UK and FD is 8903 00635