I saw the thread recently.
I have experienced the same thing in my 99 Windstar, 3.8L. Have owned the
van two years and have seen the problem several times. Happened today in
about 65°F outside temperature. Pulled over, raised the hood and cannot
find any leak. Didn't find one any other time I have looked. Have not had
to add more than a pint of coolant in the time I have owned this van.
Needle on temp. gauge stays right in the middle of the gauge. For the
record, I have been driving 50 years and do some of my own repairs.
To the OP - did you find the source of your 'steam'?