Surveying the scene eventually reached conclusion that only place it could have gone was down the open top of the ex. downpipe (big diameter on a bimmer 6). About 1 hour later having removed entire system & dragged clear of car, turned it upside down to the accompaniment of a metallic rattle as the prodigal spanner hit the floor.
Wouldn't have sounded good if the car had been started with the in-pipe spanner I guess :-/
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