As happyhippydad says, the RCD can trip with the circuit MCB off if there is a N-E fault, but the RCD must be energised from somewhere - try pressing the test button on a not-yet-fitted RCD - it won't trip. So there must be power coming from somewhere. Could the radial be getting it's live from the supply to what was the other house, and neutral from the one you are working from? (NOTHING would surprise me.) Then the circuit N would only have voltage if a) there was a load connected, and b) the circuit N was not connected to supply N or E.
Also, there must be something connected to the feed side of the RCD for there to be a path, and hence any current flowing through it.