it could be from a cracked brass fitting at the line going in the tranny leaking down to the shift selecter seal...
rebuilder should have changed it,might have,could have been damaged putting the vb back in
and there are 2 seals at the shaft,selecter shaft seal and the throttle presssure valve seal,most times the tpv seal is over looked
I would be at the tranny shop
and double check the lines,are they tight,have rubber and clamps any where in the lines
the vent would blow it out and be in the bell housing and slung around by the torque verter,looks like a front seal leak
have a dust cover on it?