Your mechanic is not likely steering you wrong as to the source of the leak. At the bottom of the transmission, the rearmost part of the bellhousing you will see the seam between the bellhousing and transmission--just in front of the pan. Clean this area well and you'll likely see it become wet with oil. A front seal will leak out the bellhousing vent openings and/or the bell to motor juncture. This rubber seal goes around the planetary gearset housing, that makes the OD ratio, and fits inside the bellhousing against the inner surface--you'll note the bell housing extends back to cover this part of the transmission. The transmission does have to come down, the bellhousing removed and the "O" ring replaced. It is a common source of leaks with this unit. Good time to replace engine rear seal as well.