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I think I get what you're saying: your coolant level actually DROPS after the engine is warmed up. This is opposite to what I have, however, yes, a picture is worth a thousand or so words.
Only time I've seen that happen is 1) bad water pump, 2) right after opening up system to change hoses, and system not properly "burped", or whatever the true car term for that is.
left for work this AM in pouring rain..wasn't about to take pix then...came home for lunch went over to truck in driveway..popped hood...coolant expansion tank was only half full....haven't driven it since last night
guess I am looking at a water pump..which isn't expensive..just a PITA to do (it appears)