2008 Dodge Grand Caravan Question: our 2008 grand caravan has a mystery oil leak ...

... and i cannot figure out where it is going. at oil change intervals (3000-5000 mi) the oil level is usually over a quart low. since the last oil change, i've had to add 3 quarts of oil. the van does not smoke. there is no smell of burning oil. i make certain that the crankcase is full of oil before leaving the oil change center. the techs insist that there is no leakage around the filter and plug. there is no leakage at all in my driveway. the van runs fine, engine hops. any suggestions?