I would check your Pressure Cap before anything else.
If you are suspecting slight coolant loss from the expansion tank then that could be the cause. Also the fact that you are at higher altitudes will cause the cap to open a bit sooner since ambient air pressure is lower.

If you had abnormal pressurization of the cooling system (ie. head gasket or cracked head) I would expect a much greater coolant loss that would be noticable in the expansion tank.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
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