If they are not hidden, it is either automatic or impossible
Question: In combat, do creatures have complete knowledge of the locations of all other non-Hidden creatures in the combat?
Answer: Generally yes, so long as normal senses apply, especially seeing and hearing.
When Alice walks into the Darkness, Bob loses track of her with his sight. However, as she has not taken the Hide action, he can still keep track of her by her sounds (footfalls, breathing, clinking armor...). Thus, he knows her location. Attacking her falls under: Unseen Attackers and Targets:
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see...
When a creature can't see you, you have advantage on attack rolls against it.
You might assume that if Bob can't see Alice in the Darkness1, then he has to guess her location. However, just hearing her is enough to know her position on the map. Even the position of invisible creatures can be tracked by sound, as explained in the Hiding Sidebar of Using Ability Scores: Dexterity:
You can't hide from a creature that can see you clearly, and you give away your position if you make noise, such as shouting a warning or knocking over a vase. An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet.
Bob knows exactly where Alice is in the Darkness, but he can't see her, and she can't see him. There is offsetting advantage and disadvantage, so he makes a flat roll in his attack.
But suppose the Darkness was in an area of Silence. At the point in time in which Alice is covered in both Silence and Darkness, then Bob would lose track of her location, even without her hiding2. In that case Bob would have to guess her location in order to make an attack; an incorrect guess would be an automatic miss, a correct guess would be a flat roll for offsetting advantage and disadvantage as before.
Now, rather than Darkness, suppose Alice walks behind an opaque wall. This is a very similar case, in that Bob and Alice cannot see one another. Bob still knows her location, however, so long as he can hear her. However, 5e does not have rules for how far sound carries, or what blocks sound. Thus, it would be the DM's decision as to whether Bob could still hear Alice or whether he would need to guess her location. At some point the wall would be thick enough or the distance would be far enough that it would effectively act like a Silence spell and Bob would need to guess her location.
Now suppose that Charlie is brought to consciousness, having not previously known about Alice. Can he detect her location? If she is merely in Darkness or behind a nearby thin wall, yes - as soon as he is conscious, he can hear her, and thus can detect her presence and location just as Bob could. If she is in Darkness and Silence, however, or behind a thick and distant wall, he cannot detect her or know her location. Furthermore, unlike Bob, he has no reason to suspect that she is there, since he did not see and hear her enter the Darkness/Silence or go behind the wall. He could, RAW, attempt to target her by guessing her square. However, since OP has clarified that there has been no communication about Alice between Charlie and Bob, it does beg the question of why he both assumes that anyone is in that general location, and why he further assumes that they are hostile and merit targeting. At least at my table, that would be considered the player using meta-game knowledge rather than the character using in-game knowledge, and would warrant a conversation before proceeding.
1'Alice in the Darkness', of course, is not to be confused with the fictitious Alice in Chains tribute band of the same name.
2Typically sight and sound are the only things that 'matter'. If other senses are in play, a DM is free to rule that Bob is tracking Alice's position by other means, such as scent.