Mystery oil leak

Hi not sure if im in the right place, new to this, ive got a vectra sri 2.2 petrol with an excess oil consumption, had cam shaft oil seals front and rear replaced, rocker box gasket replaced and still losing oil from somewhere, no visible oil leak on floor, not burning oil but putting about 2 litres a week in, subframe at back of engine is wet so wondering if under pressure leak somewhere, would appreciate any help
 
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