My 9 year old was building and painting his First Strike set with the easy build Reiver and Intercessor models and he says: "Dad! I think one of my marines is a heretic!"
In surprise I responded: What makes you think one of them is a heretic?
Son: Well I was painting this Reiver, and all the other Intercessors and Reivers have purity seals on their armor, but look! this guy doesn't!
Me: Hmm... you're right, no seal. Perhaps it was knocked off in combat, but either way I suspect the Inquisition is going to want to have a talk with this guy