Spent much of my Sunday morning debugging acad.exe left in task manager after shutdown and traced the process to a NVIDIA driver. Had the support ticket written, then found a KB. Found you can diagnose it by going into AutoCAD/Civil 3D and disabling hardware acceleration in GRAPHICCONFIG and problem goes away on shutdown.
Found a fix that if you install the driver using the Custom Install> Clean option it is fixed.
Just found a KB article this morning 
AutoCAD products continues to run in the background in Task Manager after closing
