Now, if someone has an imp under mind blank summoned, then proceeding to wipe out the mind of their summoner, would something even show up on divination?
The summoner's mind would show up as wiped to a memory search, you would have to perform a memory alteration that could stand up to scrutiny. much easier to send and invisible imp under mind black and let the summoner think they failed, though really why are you handing out eight level spell effect enchantments to imps?