9mpg is really bad but I wouldn't say fuel leak bad, I would think a leak would really show itself between uses. Did you notice the gauge reading significantly lower after the car had been at rest since the last time you saw it? Do you smell anything? See anything on the floor?
It could be something completely different wrong with your engine getting those numbers.. Engine Malfunction, fuel grade, fuel quality.. You can eliminate hard driving because I guess you would have realized that.. Any codes being thrown?
As for the DME, not sure