Toyota Repair: Coolant

Question
I have a 2007 Highlander with 45,000 miles. I had the hood open and was checking the oil today and I noticed that the coolant reservoir is almost empty, well below the L mark. There is maybe an inch of pink fluid at the bottom of the reservoir, and the engine was warm, I had driven it 15 miles or so, parked it for about 30 minutes when I checked it. Is the reservoir just an overflow reservoir, or should there be coolant in the reservoir at all times? Can I just add a 50-50 mix to the reservoir, or does this situation warrant a visit to Toyota service due to a leak of some kind? Thanks!

Answer
There is supposed to be coolant in the bottle at all times, if it goes low over a period of time there may be a small leak developing into a bigger leak, the most common on these engines is the water pump, the best way to check it is to pressurize the cooling system and do a visual inspection of the pump.