Hey @ryanmrt 

Welcome to the community!

This is definitely unexpected behavior as we have strict guidelines in place to not email customers who have not actually subscribed to your marketing. Although they did not subscribe, do they have a profile in your Klaviyo account? If so can you share a screen shot of their profile activity? It should show when the email we sent and any actions they took before/after receiving the email. It might be that they subscribed to the list to get the discount and then unsubscribed. Another idea if they don’t have a profile, is that someone who has subscribed to your list forwarded the discount code to them for use. 

Let me know how else I can help!