I didn't open any Opening Day in 2013, so I can't say for certain. The only site that I can find is this:
http://www.sportscardradio.com/2013-topps-opening-day-baseball-checklist/
that lists the set in the checklist. I do know that Sports Card Radio uses the Topps Press releases for its checklists, and it wouldn't be the first time that Topps had an unnanounced insert set. At the same time, the Cardboard Connection site doesn't list the insert set, and they also use the press releases. So, not really sure. I can't find anything linking it to another set as an insert, so my guess is that they were actually in Opening Day packs. Short of buying a box, I can't be certain.
Always looking for baseball variation/error cards and anything Garrett Whitley or Ian Anderson