Threats Undetected sends the unused card(s) to the bottom of your library. However this still triggered Curse of the Veil which states it deals damage when a card goes to graveyard.
@TheHunter the actual card you cast, in this case Threats Undetected, goes to the graveyard once it's finished casting 👍
Haha, I was so affronted at getting pinged for 3 I completely ignored this . Heinous accusation of bugginess withdrawn!!
@TheHunter Thank you for the withdrawal - it truly added a laugh to my day.
Lu