A Level 14 College of Whispers Bard acquires the following class feature:
Know of the shadowAs an action, you magically whisper a phrase that only a creature of your choice within 15 yards of you can hear. The target must make a Wisdom save roll against your spell and save DC. It succeeds automatically if it does not share a language with you or if it does not hear you. On a successful save roll, your whisper sounds like an unintelligible murmur and has no effect.
If the target fails its save roll, it is charmed by you for the next 8 hours or until you or your allies attack or damage it. He interprets the murmurs as a description of his most mortifying secret.
Although you have no knowledge of this secret, the target is convinced to know it. Although charmed in this way, the creature obeys your orders lest you reveal its secret. He will not risk his life for you or fight for you unless he was already inclined to do it. He grants you favors and gifts that he would offer to a close friend.
When the effect ends, the creature does not understand why she kept you in such fear.
(p.16, Xanathar's guide)
When using Shadow Lore, does the bard physically whisper something that only the target can hear, or is it more of a telepathic murmur than the target hears?
Could the target, or a nearby creature, say that Shadow Lore was used and potentially who used it?