prism,

I think you have the quite common 'mysteriously disappearing coolant' (MDC) issue. I have topped up coolant every 3-6 months, sometimes added as much as 1 litre to get that little floater back to level. Very odd but no other symptoms day-to-day.

I recall others here having MDC, Ol Motor Dave in particular. So MDC appears to occur on 3.0 and 4.4

I did have a complete failure on the coolant reservoir cap; but that was dramatic, rapid and obvious, OBC complained and then steam from under the hood were good clues that something was amiss ! :-)

Scotty