There is usually just a slight groove worn into the metal. If you were not looking for it I suppose it could be overlooked. That one bolt on the pump could also be the culprit if not sealed up when installed. Is the oil pan sealed well also?
Just thinking outloud. <img src="/forums/images/graemlins/cheers.gif" alt="" />

1988 4Runner V-6 Auto, Locked out back, LSD up front, some lift, some tire, and a casual attitude towards body damage.