Very low coolant level with no evidence of leaks

Question regarding 99 Dodge Caravan (very low mileage, <90k). My wife heard a ding (not sure which light came on) and noticed that engine temperature was very high. She pulled off and called me. No coolant leaks were visible but the coolant reservoir level was low out of sight. I told her to let the engine cool down and drive to the next gas station about a mile away. She filled the reservoir to max level (added about 1/2 gallon of 50/50 antifreeze) and let run briefly. No leaks noticed so she drove the short distance home. When I checked it a little later, coolant level was again low out of sight. I added most of the remaining half gallon to the reservoir and noticed bubbles coming up (I assume this was just air escaping as the system filled). The next morning the level was down a little bit. I topped it off and took it to work. The level looked good when I arrived (a little above max with the engine hot) and was back down to max level when I came out to go home. I drove again today and everything looks good and the engine runs cool where it normally does. Where did all the coolant go? I checked the oil dipstick and oil level was normal with no evidence of coolant or contaminated oil. I know I need to take it to get checked out but I am invisioning a very expensive witch hunt to find the problem. There is no evidence of any kind of leaks on my drive way.