Not sure of this, because I assume only a tiny fraction of people does this, and those who do probably aren't consistent. E.g. for Amazon Prime, some might use "+amazon-prime", some "+amazonprime", some "+amazon" etc., so there would be very few overall repetitions even in a large data set.
Right but grepping for "+" in emails is also high on the list of things I'd do to identify an unknown information dump. Given that he's used to dealing with those I'd be surprised if he hadn't thought of that, although it probably doesn't hurt asking him if he did try it.