Later consultation with the Silicon Valley Wildlife rescue web page suggested that opossums under seven inches were too young to be on their own, so I went to check on it. Still wandering around the yard, and limping a bit. Given that the sun was going low, and my yard has at least three cats that visit at night, I got my housemates to help winkle it out from under my car and swooped it off to the Humane Society, which does night intake for the SVWC.
Of course, I just now have found a site that suggests that possums 4 to 6 inches long are fine on their own, so it's possible I rescued a perfectly healthy non-orphan. But I'd really like to avoid Cinder coming in at night and showing me the very large mousey she caught.
No pictures, because I didn't think to do so when it was wandering around the yard, and once I got it to the shelter they didn't want me to unwrap it and stress it.