"Once" implies emergency. When you use "once," the event that follows happens immediately. "After" just implies "sometime in the future," so use it in all other cases. For example: "Once I touched the hot stove, I screamed." versus, "After I put the food in the microwave, I was eating." One happens a little more immediately than the other. On a side note, your past tenses aren't consistent. It should read "Once/After Susan had left Officer Thompson, she came across a prostitute on the corner."

"After" would be more proper from a scholarly perspective, like writing a paper. The main reason is because "after" simply has less meanings than "once" does, and it makes for easier translation. "Once" would be more appropriate when writing a short story, or fictional book; ex. "Once upon a time".

Supporting the two explanations. And yeah, jkduzz is right. The statement should be parallel. If it starts with past tense, the rest should follow.