Answer: had been looking
Explanation: “The past perfect continuous tense is used to indicate which of two actions that occurred in the past was ongoing for a relatively longer period. When the two actions are linked by ‘before,’ the verb that comes after ‘before’ is in the past indefinite tense, and the verb that comes before ‘before’ is in the past perfect continuous tense.”

So the correct answer in the above sentence would be “had been looking.”