Low coolant level in 2.7

I changed the coolant in my '02 2.7 in October. Everything seemed fine, then today, I was poking around under the hood and noticed that the coolant reservoir was rather low (I put relatively few miles on my Taco and am not under the hood that often). Took maybe 8-10 ounces to bring it up to the Full line. Could something be wrong, or maybe is it just that the heater core never got filled up when I did the refill? I did run the heater after I did the refill to try to make sure it got full. I've never noticed anything funky in terms of the exhaust smell/color/etc. Perhaps there's an ebb and flow in terms of what's in the overflow reservoir at any given time...