I have an ongoing problem with heater cores, on my 5th right now. I noticed a small puddle on antifreeze under the truck a few months ago but since that time I have been watching religiously and constantly checking the undercarriage of the truck, nothing, not even a drop! However, I have noticed that my coolant level is down (about 2" in the reservoir) and slowly going down. Also, I haven't had the steam out of the vents that occurred when the other heater cores failed. Where else could it be going? What could leak and then stop leaking? Closed system means level shouldn't change.

 

You could of had an "air bubble" in the system from the previous repair, and it "burped" out the overflow....... if the fluid level continues to decline, do a compression check. (headgasket).

 

most recent repair was almost 1 1/2 yrs ago.