I'm going to say something daft now..

BUT..

I had an almost identical staining leak under my bell housing. It would drip onto the floor and stain the alloy collecting dust, at almost the same point.

Stupidly enough, it was a leaking cam seal behind the front cover..!! It would weep and the airflow wind would blow it underneath until it reached the bell housing, where it would drip.

I also stuck a finger in that hole...it was bone dry...

Good hunting..

Steve