The hold mode you're referring to is different than the one discussed previously in this thread. The previously discussed mode initiated by a brake press shows an H in a circle in the upper right of the dash and pressing on the accelerator results in normal driving. This one says the word "HOLD" in the middle top of the display and pressing on the accelerator resumes AutoPilot or TACC (depending on which you were in). When TACC first came out (before auto-steer) it would enter hold mode frequently. It would happen after stopped for either 10 or 30 seconds depending on the conditions. More recently, the only time I see this hold mode is when I enable AP just as the car is slowing down. When it stops, after a couple of seconds, Hold is activated and you have to press on the accelerator to resume.