There's a chemical test to rule out exhaust gas in the coolant. Head gasket leak will go both ways and much more pressure in the cylinder than in the coolant.

Most missing coolant without a drip are from a slow leak that evaporates.

In my case those have been the radiator outlet sensor and the reservoir seam. I've fixed both of those on both of our e53.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)