2011 BMW 328i low coolant warning every two weeks

That's a textbook N52 slow leak, and the fact you never see a puddle tells me it's small enough to evaporate on hot parts or hide on top of the belly pan.

First place I look on these: the plastic coolant outlet flange on the front of the cylinder head where the upper hose meets the thermostat housing (the "Mickey Mouse" fitting). The o-ring hardens with heat cycles and it seeps just enough to trip the sensor every couple weeks.

Second: the expansion tank. Hairline cracks in the plastic are invisible cold and open up at operating temperature. Check the seam and the lower hose connection closely.

Third: the radiator end tanks and the vent nipple on the upper radiator hose. A tiny seep there gets blown back by the fan and leaves a white or pinkish crust instead of a drip.

The right move is a cooling system pressure test, done cold, and again hot if nothing shows cold. Pan off, tester on, watch it for ten minutes with a flashlight. Do not throw parts at this one at a time.

Keep it topped off with the correct blue BMW coolant, not universal green. At 133k, if that flange and the expansion tank are original, they are due anyway. If the pressure test holds perfectly and the level still drops, then you are looking at internal consumption, which is a different and bigger conversation.