same problem..but mine is obvious leak coming from the top most of my radiator.. uhmm first try those mechanical style like checking if you've got busted water pump or head gasket.. if my bubbles or wla.. 2nd maybe hindi mo napansin na hindi mag on yong aux fan mo or minsan meron fan or minsan hindi so mainit na sya and does your temp goes higher to its normal level? 3rd maybe a small bleeding under the radiator or wherever side.. 4th maybe radiator cap not properly tightened or needs to be replace.. 5th tama ba yun level ng reservior mo at hindi mo pina overflow kpag mag refil ka? just troubleshooting if may suspect ka.