My cars oil keeps disappearing?

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I drive an SUV (2005 MB ML350 3.7l) with 115k miles. The other day my yellow oil light came on. I pulled over and checked the oil level which was very low (1/4 full on stick). I added a few quarts of synthetic blend and drove away. Now about 6 weeks later, I checked the stick and the oil is low again. I smell no burning and see no leaks. In fact about 3 months prior to this, I took it in to my mechanic for an oil change and they noticed a leaking valve cover gasket and oil pan, so I had all the gaskets replaced (which was covered under our warranty). The car seems to run fine besides this, no noise issues. My question, where is the oil going and why is it disappearing?