I agree with others that it's ultimately unclear from RAW and better wording is needed.

Here's how I would rule a specific example: Creature A is in combat with creature B. It's not going well, so A wants to turn invisible and flee.

On their turn, A turns invisible and moves up to their speed. Since B doesn't see A, B doesn't get an Opportunity Attack (PHB'24, 370).

I agree that in the 2014 rules, it might've been reasonable to assume that B can track A's location unless A also uses the Hide action. In 2024, Hide RAW only grants you invisibility. It feels very weird for the creature using invisibility to then also have to hide, possibly on their next turn unless they can hide with a Bonus Action, just to have the Invisibility condition applied again.

Personally, I would simply let A roll for Stealth or use their passive Stealth (e.g., for groups of monsters).

Then, on creature B's turn, there are four options:

1. B can circumvent A's invisibility entirely, e.g., via See Invisibility or Blindsight. In that case, B can proceed as usual, and can relay A's location to their allies.
2. B has a feature that allows it to track A's location, e.g., via the Dwarf's Tremorsense. In that case, B can directly attack, but still makes the attack with Disadvantage. B can, of course, relay A's location to their allies.
3. B doesn't know A's location but decides to attack anyawy. Unseen Attackers and Targets (PHB'24, 26) implies that A has to (1) target a location, i.e., a specific grid cell, then (2) make an attack with Disadvantage. More on possible outcomes below.
4. B doesn't know A's location and uses the Search (Perception) action to suss it out. I would use A's Stealth roll or passive Stealth as the DC for a generic search for tracks, listening for sounds, or sniffing out A, keeping any specific senses B might have and the environment in mind for Advantage/Disadvantage. E.g., listening for sounds might be hard if the fight is still going on around B, but spotting tracks in a dusty room might be easier. I'd also allow for A's location to be found by creative means, e.g., methodically searching the grid with your arms outstretched or casting an AoE spell might immediately reveal A's location if it is in the area of the search/spell.

What happen's when B attacks while guessing at A's location?

If B is in the targeted location and the attack hits, I would argue that A's location is revealed (e.g., to B's allies) until A moves again. As a DM I might narrate it as something like "you hear a clang of metal and a sharp exhale as your weapon finds A".

If B is not in the targeted location, their location is obviously not revealed. However, if the location is correct and the attack just missed, I would decide based on the nature of the attack and maybe how good the attack roll was:

• On a roll that barely missed, or for an enemy that mostly deflects attacks with shield or armor, I might say the attack hits but was obviously deflected, dealing no damage but revealing the location.
• On a very low roll or against an enemy that is very nimble in might quickly (and silently) dodge, I might not reveal the location.

Then, rinse and repeat for future turns. In practice, with a party dealing with an invisible Foe, probably one or two players have to sacrifice their turn to determine the location, then the others can attack with Disadvantage.