[Lord of Hog]

This may sound daft but the smell of OAT is pretty unmistakeable. Get the car out of the wind and preferably in an enclosed space, stick your nose under the bonnet and have a good old snuffle. If there's a trace of it in the V you'll notice it (particularly with a slightly warm engine).

Secondly please remember that a lot of the points with the greatest potential for coolant loss are a) difficult to see and b) warm, meaning that your evidence is going to be nothing but a pink stain and will be hard to see. Hence the usefulness of the sniff test.

Thirdly, based on my own experience if it's not the thermostat O-rings, the thermostat straight pipe, the inlet manifold, the radiator or the plastic elbow on the radiator it's likely to be an air lock. Some of the coolant channels inside the engine are extremely narrow (hence their susceptibility to clogging with RadWeld and such) and can take a long time to give up any air that enters.