You might be right! I don't have the other expansion (Unseen Forces) but it might be designed to work with that.
Yep, it is part of the blessing/cursed ruleset of Unseen Forces, but it's for getting blessed, not cursed. Getting cursed looks way more sinister than the angel, if you can believe that. Here's a pic of the Blessing and Cursed cards from UF and kinda fit in with how the add the Yellow/Red dice cards look (Where Yellow has the 4Investigation symbol and Red has the Wildcard symbol, the Blessing card has the Blessing reward/penalty icon on it and the Cursed card has the Cursed icon)
It's the blessed symbol from Unseen Forces. You can total just ignore it if you don't won't to buy another expansion.
In fact, you are FORCED to ignore the blessed/cursed symbols if you don't have UF or don't want to play with the Blessed/Cursed system, when the adventures have the choice reward/penalty (The big / Symbol between the sets)