The whole poem is a single sentence, and its surprise is in the last word. "We drive through a small rainstorm that seems lost, too." That "too" matters. Before we reach the rain, it quietly tells us that the people in the car already feel lost, or at least a little unsure of where they are. We never get an explanation, which makes the feeling more believable.
The title suggests a forest road, a place where it's easy to lose your bearings. The rainstorm is "small," so it isn't a threatening storm. It's something almost endearing, a patch of weather that has wandered off from the rest of the sky. The speaker looks at it with gentle humor and a bit of sympathy, as if it were a companion.
The mood is quiet, wry, and slightly wistful. Being lost here isn't frightening. It's shared, and it's softened by noticing something odd and lovely outside the window.