Would the Arcane trickster casting the spell need to be unseen in order for an invisible Mage Hand Legerdemain to go unnoticed?
Say for an example in a conversation a rogue wishes to pickpocket whoever it is they are talking to - would they need to cast the spell before the conversation starts? Could the spell be very slyly cast without the 3rd party noticing in any way?
The Mage Hand Legerdemain feature description says:
Starting at 3rd level, when you cast mage hand, you can make the spectral hand invisible. and you can perform the following additional tasks with it
So you need to cast the Mage Hand normally. The spell description says:
Components: V, S
That means the spell has Verbal ("the chanting of mystic words") and Somatic ("a forceful gesticulation or an intricate set of gestures") components. You can't perform them unnoticed during a conversation. If the other person identifies the spell itself, depends on the person.
Related questions:
It remains unclear though, how obvious is a process of controlling the hand. The spell description only says that
You can use your action to control the hand.
So when you control it, you must expend an action (or bonus action, for the Legerdemain version). Probably you have to perform special gestures, sort of. The exact details depend on the DM.
Quoting a similar response for Charm Person:
"Under Spellcasting…Targets, the PHB states that spells with subtle effects can be cast without being noticed:
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless the spell says otherwise.
(PH, p 204, emphasis added)
Even the target typically doesn't know
The casting of charm person is subtle enough that even the target doesn't know about the spell until it expires.
When the spell ends,the creature knows it was charmed by you (Charm Person, PH 221)
If even the target of the spell is unlikely perceive it was cast, other observers would have less of a chance. (A court magician, trained in Arcana and watching for such things, would be a different "
That being established, (thank you for the foot work Tim Grant) I would look at what could be considered a "subtle" spell.
It would be up to your DM to decide how subtly Mage Hand can be cast, assuming that a low level spell would be easier to cast/require less movements than a higher level spell with more obvious effects such as fireball.
Personally i would argue that a Sleight of Hand vs Passive/Active Perception based on the scenario would be the way to play it. I feel that Mage Hand, as a cantrip, and using those cited references, would qualify as being a much more subtle spell. Without being able to cast it undetected it greatly limits the usage of Mage Hand Legerdemain to the point of "why even take it".
