In the previous part of the text the author says that a taxi driver picked up a young girl. While he was driving, he looked at the rear mirror but he could not see the girl. Then there is this sentence: おや、変だぞと振り返ると乗っている。 What does it mean?
This is very similar to a question you asked the other day. So, I will assume you're having difficulty parsing the complexity of the grammar.
First let's break the sentence down into its three main parts:
Whoa! That's strange!
Looking to the back (technically, "turning back")
She was riding [there].
If you can see the meaning of those three parts, then all you need to figure out next is the nature of the two と's, which function quite differently grammatically here. But they're the glue that join these parts together into a meaningful unit.
The first と is quotative. The implicit verb omitted here is おもって, "thinking". So the first portion can be translated (a bit freely)
Thinking to himself, "whoa, that's weird!"
The second と is a conditional, which here means "when" or "upon".
When he looked in the back (or, upon looking in the back seat)
And then you're just left with,
There she was (technically, she was riding [in the back seat])
I'll presume that he's picked up his fare (the young lady) and then he doesn't see her in the rearview mirror. At that point, the sentence should make quite reasonable sense. (I fill in a few details that are either suggested by the Japanese or suggested by implied context.)
"Whoa! That's weird. [I thought just picked up a passenger]" But, as soon as he looked in the backseat, there she was.