It's quite possible with a minor leak. I had that with an old BMW 733 I owned over 20 years ago. I finally figured it out when it got so bad that the coolant system wouldn't pressurize. At that point it was all leaking into the engine. Before that, everything tested fine (pressure-wise, nobody did the oil and coolant tests), but the temperature ran up and down.

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'96 C280 (gone)
'97 C36
'05 C230k