Yeah, that's what I was thinking too. I haven't touched it for a while, but it may have been due to air bubbles in the system. I'll see if it keeps happening and report back.

The other issue I still have is that the oil on the dip stick is well above maximum. We drained all the old oil out and added 4 liters as per the sticker on the head cover. When the engine goes near operating temperature, the oil light and buzzer start going off. Is it possible that the oil thermostat isn't letting the oil into the radiator?