loss of coolant, no visible sign , no contaminate eng.,oil/ no white smoke from exhaust ? peugeot 307 sw hdi

Topping coolant level on a daily basis...no water in engine oil / no white exhaust smoke / no visible signs beneath car/had car on ramps with engine running when topped up ..STILL NO LEAKS ! car runs fine, barring STOP/CHECK COOLANT sign lighting up on dash