Howdy Phil,
I'm less worried about short ground visits as long as the critter goes back up afterwards with no signs of any trouble. I have a panther that basically takes a daily tour of his 2x2x4 enclosure, top to bottom just about every day. If, on the other hand, he stayed down on the bottom for hours or didin't return before bedtime to sleep up above, combined being on the bottom with any signs of illness (sunken eye etc.) then you have a problem on your hands. Spotting health issues can be difficult with chameleons and are often discovered too late for recovery possibilities.