[RPG] Do you have to hide in order for other creatures to not know your location inside a fog cloud

The creature can't be seen, but they may be heard or smelled

The creature doesn't have to hide, but hiding will help to take advantage of the fog cloud's obscurement. If creature wants its location to be "unknown" hiding becomes necessary.

In this case, if a creature doesn't hide can know where they are for the purposes of a melee or an AOE spell attack?

They can, but AoE can be tricky, and how the target is detected will vary.

Melee attacks

The problem for the melee attacker is getting into melee range inside the 20' radius circle so that the attacker is within melee range even though it can't see the target. A creature would expect to have disadvantage on such an attack roll, or even to miss by not being close enough to make a melee attack.

Unseen Targets (Chapter 9, 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.
If the target isn’t in the location you targeted, you automatically miss, but the GM typically just says that the attack missed, not whether you guessed the target’s location correctly.

AoE attacks: Depends on the AoE's radius and effects

AoE attacks are a bit trickier to adjudicate, since AoE varies.

If the AoE is big enough, the size of the fog cloud may not suffice to prevent the radius of the AoE's effect from hitting the target inside the fog cloud. As an example, if my dwarf Tempest Cleric is inside of a fog cloud with a 20' radius, and the enemy Archmage casts a fire ball at the edge of the cloud (which is a point that he can see) I am by intersection of those two circles in the AoE of his fireball. {roll save for half damage} If, on the other hand, I had cast that spell and stepped back 10', I'd not be in his fireball's blast radius. But if he had a spell with a 40' radius, I'd still be in the AoE if he aimed it the same way as the fireball example. (As a quick example, frozen sphere has a 60' radius, so my Tempest Cleric would be subject to its effects if the Archmage cast that spell).

From Hide (Combat, Chapter 9)

When you take the Hide action, you make a Dexterity (Stealth) check in an attempt to hide, following the rules for hiding. If you succeed, you gain certain benefits, as described in the “Unseen Attackers and Targets” section.

Fog cloud provides obscurement

Let's walk through the spell and how "obscurement" works.

Fog Cloud Duration: Concentration, up to 1 hour You create a 20 - foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured.

From Vision and Light section in the rules:

A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition (see Appendix A) when trying to see something in that area.

Creatures that rely on sight won't see whomever is in the fog cloud. But other senses are involved in Wisdom(Perception) checks, so a "Hide" action may help make detection by other senses more difficult. (Hearing, smell, etc.)

Blinded (Appendix A)

• A blinded creature can’t see and automatically fails any ability check that requires sight.
• Attack rolls against the creature have advantage, and the creature’s attack rolls have disadvantage.

If the creature trying to find the character in the fog cloud has to rely on sight, then they cannot see them. If other senses are being used, then an attempt may be made to detect the hidden creature.

How else might the creature in the fog cloud be detected?

Smell and hearing, or abilities like tremorsense or blindsight.

Perception. (From "Using Ability Scores")

Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest.

There are a number of monsters that have advantage to perception checks that rely on smell. For example, Cloud Giants and Werebears have:

Keen Smell. The {giant/werebear} has advantage on Wisdom (Perception) checks that rely on smell.

Special sensors will modify the above

There are monsters like Umber Hulks who have other senses that let them find you without sight.

Tremorsense

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance. Tremorsense can’t be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs and umber hulks, have this special sense. (SRD, p. 257)

In a case like this, hiding in a fog cloud by staying very still could give disadvantage to a creature using this sense to find another creature, or lead to "Unseen target" depending on how your DM rules "vibrations" ... is a heartbeat enough vibration to sense? That ruling may vary from one DM to another.

Blindsight

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as grimlocks and gray oozes, typically have this special sense, as do creatures with echolocation or heightened senses, such as bats and true dragons. (SRD, p. 257)

This would render the "hide in a cloud" effort moot.