Q: I'm losing coolant, but I don't know from where

My van is losing coolant, but I can't figure out from where. The timing belt and water pump were changed around 100,000 miles. I usually change the oil every 5000 miles or so. I waited a little longer than usual on this - almost 7000 miles. Sometime between the last oil change and this one, my coolant reservoir went from the halfway mark all the way down to the "add coolant" line. It was at the halfway mark every since the timing belt change, and until now the level as never budged. There aren't any leaks that I can see. Two oil changes back, I had an oil analysis done, and I was told there were some indications of possible coolant contamination in my oil. However, the brand of oil I was using sometimes has those particles in it, and the percentage is so low it's not supposed to be anything to worry about. I'm having the oil tested again, but are there any other common areas I might lose coolant that wouldn't be very visible?