D&D 5E How do you handle someone who is not surprised but is unaware of any threats?

[clearstream]

Investigator Rogues and Rangers can Search as a bonus action (enabling them to find the enemy, and attack as an action). Pretty sure there was an UA Skill feat that let you Search as a bonus action as well.

Sorcerers can Search as an action, and then cast a Quickened spell at the detected enemy. Fighters can Action surge [Search + Attack]

There are a few other ways to do it as well.

I hadn't thought about the Search+X set of actions - great thinking!

Also - 1) Move to an advantageous position. 2) Try to Hide. 3) Dodge. 4) Cast a buff or defensive spell. 5) Cast an AoE to shape the battlefield. 6) Flee. 7) Ready a ranged weapon to shoot the first creature you see attack. 8) Ready a spell to...

 

[Imaculata]

I would probably narrate it as if the player with Alert heard a sound or saw something that the rest didn't. A snap of a twig or a reflection from the shadows. Just enough time to draw a weapon, cast a defensive spell, move to a new position, or ready for battle. Keep in mind that while the game is turn based, in the fiction all of this is happening in a blink of an eye.

 

[NotAYakk]

The cut between non-combat and combat doesn't happen on an action break. It happens anywhere.

Combat starts when it starts. Whatever happens outside of combat, happens, but it might not finish.

So suppose you are chatting, and suddenly one character draws a knife. They might be able to draw that knife and attack in one action, but combat starts with them grabbing the knife and starting to draw it.

Or they already have the knife, and they are bluffing they aren't going to attack. They attack, with a successful deception roll. Combat starts as they push the knife towards the target.

Or they cast a subtle hold person. Combat starts as the target feels their muscles lock up, but before the spell finishes. They might not know who is casting hold person.

Combat can start with arrows in the air, combat can start with a flames licking out of someone's hands from a burning hands spell, combat can start with a knife being drawn, a fist being pulled back to punch.

The non-surprised person can get a full set of actions in that fraction of a second between the arrow being in the air and it hitting.

Remember, the "bob acts, then alice acts" of 5e combat is an abstraction. If bob moves with the arrow in the air behind cover, then the arrow misses because of cover, that means bob ducked behind cover. If it hits, it means the arrow could have hit as bob was moving, but maybe didn't explode or kill bob until after he got behind cover.