A few weeks ago I noticed when I moved my '99 3.0 Ranger that some odd fluid had leaked onto the pavement. I wasn't totally sure what it was and I checked around underneath to see what may be wet. The only obvious wetness was from years of valve cover gasket leaks.

The shop that rebuilt the transmission was replacing a faulty valve body they installed the next day, so rather than risk destroying something by driving with possibly low fluid, my girlfriend and I had AAA just tow it. 

The shop replaced the valve cover gaskets, but said they didn't see anything else that recently had leaked. I noticed on the receipt that they wrote coolant was 18% low and they topped it off for free. They didn't verbally mention this discovery. Fast forward to this evening, I put a bottle of rain-x into my washer fluid reservoir and recalled the note on the receipt. I checked the coolant reservoir and it was empty. I drove a short distance to the parts store and put coolant in, as I had no other transportation. It didn't take much to fill back up, luckily. I also put on a new radiator cap in case that was bad.

I checked it again after driving long enough for the truck to reach operating temp. I noticed an intermittent bubbling in the reservoir. The dashboard thermostat has never indicated an overheating situation, and the heat has never seemed a little on the cool side, which would indicate a loss of coolant, so nothing has seemed out of the ordinary to this point. Could this bubbling be a sign of head gasket failure? The mysterious leak has not re-appeared since that day, my radiator was replaced two years ago, and the cooling system still seems to be in good shape. 

Thanks for any thoughts/ opinions.