I had wondered that myself, the radiator and hoses all seem to be in good condition. There is a possible leak on the thermostat housing, I saw a bit of seapage there when it got hot, it seemed to be burning off as it came out. Over time it could have came out here, also it may be leaking near the water pump, if they are like others I have worked on there is a vent hole that leaks when the bearing goes? I'm hoping there is not a headgasket issue, although it looks like a pretty easy engine to work on. If the gasket is bad will the head warp?

I am optimistic about the headgasket since the oil is not milky and the exhaust has no smoke.....