Where did my coolant go ?

i regularly change my oil with synthetic and always check all the fluids. this morning the low coolant light came on, when i checked the coolant was nearly empty in my reservoir tank. this is strange because i just checked it yesterday and it was good. Where could have my coolant gone? nothin on my driveway and the oil doesnt look like it is contaminated. THe engine has never over heated or anything. THis past summer i had topped off the coolant with some distilled water