They weren't testing the vehicle at that point anymore. The Maus project had been cancelled for many months at the time. Christian
Fair point! I do reckon that if they had had fuel & ammo they would have used it, even just as a pillbox. However, the surviving example does not show any 'battle damage', and it seems unlikely that it did have fuel/ammo, so...
The germans called them S-Boats, for Schnellboats (Fast Boats). The Brits named them E boats, standing as you correctly said for Enemy.