Cadillac Repair: Leaking Coolant, plastic jug, coolant level

Question
I have a 1997 Cadillac Deville and have had work done several times dealing with coolant.  But every so often the Check Coolant Level light always comes back.  I don't really see it leaking from the bottom nor do I see smoke coming from my tail pipe.  I recently had my mechanic put pebbles in the radiator as a sealant he says.  Well it work for almost two weeks, then the same problem.  I've had a brand new radiator, thermostat and heating cord.  Please tell me, what else could it be if he says there aren't any leaks from the bottom and me oil isn't milky so he doesn't think it's my motor.

Answer
If the radiator and the over flow plastic jug are staying FULL of coolant then the sensor has an intermittent fault and needs to be replaced. Buy a new one so you know what your looking for. It is most likely located in the overflow plastic jug or is a thin black plastic single wire plastic sensor screwed into the side of the radiator.