It is quite common for radiator caps on XTs to fail and open and vent coolant at too low a pressure, or even permenantly stay open. In fact there was (a good while back) a batch of bikes that were supplied with faulty caps.

It sounds like you're losing coolant when the engine reaches operating temperature and the system tries to become pressurised (which it should do) and it can't because of the cap. This will also explain why your fan is late cutting in (or not at all) as without being pressurised the cooling system can only reach ~100C; under normal pressure it can reach 112C. The fan switch/thermostat cuts in at 102C, so clearly if the systems not holding pressure it won't reach that point.

In a nutshell... try a new radiator cap!

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