(native speaker) I agree. The original with "been being watched" feels like something the detective says when he first discovers the fact about watching. It ha

(native speaker) I agree. The original with "been being watched" feels like something the detective says when he first discovers the fact about watching. It has an exclamative aspect to it. There's a present sense of realization, mixed with the past sense of when the watching had taken place. "She would have had to have been observed." is something like a lawyers distillation of what happened later for the court.