The Arcane Archer fighter (Xanathar's Guide to Everything, p. 28-30) gets a number of Arcane Shot options, allowing them to unleash special magical effects when they fire a longbow/shortbow. One of their Arcane Shot options is Banishing Arrow:
You use abjuration magic to try to temporarily banish your target to a harmless location in the Feywild. The creature hit by the arrow must also succeed on a Charisma saving throw or be banished. While banished in this way, the target’s speed is 0, and it is incapacitated. At the end of its next turn, the target reappears in the space it vacated or in the nearest unoccupied space if that space is occupied.
If a creature gets banished in a group of enemies and no one sees the creature get banished, do the other creatures notice he is gone and are still surprised if you decide to attack them next round?
(Obviously before the creature comes back at the end of its next turn… although THAT might get tricky depending on initiative rolls and when the DM states "it's now this guy's next turn", but for the sake of simplicity, we'll say that because you don't act on a surprise round he doesn't get the first turn when he's hit.)
As far as I can tell this is really a matter of timing. If you hit a creature and it's able to do ANYTHING (even saying 'ow') before being banished, then it's obvious that this is your surprise round and you don't get to grab another one… On the other hand, if the creature is banished THEN gets to say 'ow', well, no one can hear him; and if no one saw him go "poof", then you're likely to be able to just get another round out next turn with people still not knowing you're there.
Best Answer
Your Attack Reveals You
I am not sure whether a creature getting banished makes any noise. It's possible that the creature disappears without any sound, or that it vanishes in a flash of light screaming. Either way, that isn't what will give you away: it's the attack you needed to make to banish the creature.
Banishing Arrow states that:
Since you need to hit a creature to banish it, you needed to make an attack. However, the rules on unseen attackers and targets state that:
Note that it doesn't say you give away your location only to the target of the attack. When you attack while Hidden, you lose your Hidden status. Naturally, you wouldn't become visible to someone miles away, but basically anyone who could have been alerted by an "ow" from your initial target has already been alerted by your attack. (You can't even get around this with the Skulker Feat, because your attack needs to have hit).