From what I can tell, Jennings, the Me-262 slats were counterbalanced and spring loaded. They would handily fall into the open position on the ground, barring any reason not to. But they could also be rather easily pushed back into retracted position by attendant ground crew and locked there.
Im guessing as part of pre-flight routine, they were opened and allowed to function normally, i.e., automatically. This explains the videos Ive seen of the fully restored model and the deployed slats during pre-flight run-up.
Its also why I asked here, among the cognoscenti.
On a personal note, I remain mystified how a kit maker would overlook this feature, even with all the fine explanations. It is almost too prominent to miss, IMHO.