check engine light comes on and off as well as radiator light.help!

ok a while back i wrote a post about my check engine light coming on on my 2002 325ci .i ran the code and it was my coolant level, thermostat or a sensor.i filled my coolant and the light went off.every once in a while it will come back on and go off maybe take 2 weeks to go off again but then will stay on for a day and then shut off.recently my yellow radiator dummy light pops on and pops off.now i add coolant to where the floating pin should be at the filler neck.then the light will come on again maybe a day or two later and the floating pin in the filler neck will be at the bottom.where is my coolant going?i dont see any leaks.im a little confused was thinking about doing a complete coolant flush and having them put in the bmw coolant just to be on the safe side.any advice or help would be greatly appreciated guys thanx