it's normal to loose coolant over time even a signiffigant amount water (50% of coolant) evaporates!! in the summer months especially. your supposed to check your tire pressure, coolant level, steering fluid, brake fluid, at least once a month u never know when a real leak might pop up. there may be a coolant leak present; however, only way to know is a coolant pressure test. as long as its not leaking through the head gasket it shouldn't pose a problem just keep an eye on your coolant I check mine monthly sometimes it's right where I left it sometimes it's almost emptied the resevior. when I changed my timing belt I found the water pump I have a small leak where it seals against te block. I suppose I was loosing some there but not enough to lose pressure in the coolant. replaced the water pump and seal and I still check and keep up the coolant levels they still vary about the same hope this helps