This should read “como si esperaba a alguien.”
“como si” is regularly used with the past subjunctive, so I would say that “esperaba” is wrong.
Como si triggers the subjunctive because what is being described in not real, e.g. “He felt as if he were ten feet tall”. He’s not really ten feet tall, is he?
In the original cloze sentence, the person is looking around as if he waiting for someone. Here, the narrator is describing an action but isn’t sure what the actual cause of the action is. If the narrator knew for sure the cause, they wouldn’t use the subjunctive, nor the phrase as if.