According to Longman English Grammar by L. G. Alexander,
"Adverbs often associated with the Present Perfect: already, ever, for (+period of time), just, never, never...before, since (+point of time) are often used with the Past Perfect to emphasize the sequence of events:
When I rang, Jim had already left.
The boys loved the zoo. They had never seen wild animals before."
Unfortunately, the author didn`t provide an example for since.
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