99 Park Avenue Disappering Antifreeze

Hello group, I fear the worst of things, My 99 Park Avenue has 114k miles, I had the Radiator flushed and refilled with the DEX Collant, SInce then It
seem like the Antifreeze disapperars, I filled it up to the coolant line.Within 3mnths time its empty. I check the radiator its full. I see no leaks, no change in cars driving or idling. I assume the guys would tell me if there was something found in the oil when its changed. This cars has the 3800 engine. My mother inlaw has the Lesabrea 2000 model hers the same way.. Does this engine use coolant in such a way.
THank Jay Smit
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.