BTW: the DM granting advantage or disadvantage based on the current circumstances is in the section on using ability scores, not under modifications to the game. The example they give is:
For example, imagine a wizard is running down a dungeon corridor to escape from a
beholder. Around the corner ahead, two
ogres lie in wait. Does the wizard hear the
ogres readying their ambush? You look at the wizard’s passive Wisdom (
Perception) score and consider all the factors weighing on it.
The wizard is running, not paying attention to what’s ahead of him. This imposes disadvantage on the wizard’s ability check. However, the
ogres are readying a portcullis trap and making a lot of noise with a winch, which could grant the wizard advantage on the check. As a result, the character has neither advantage nor disadvantage on the Wisdom check, and you don’t need to consider any additional factors. Past encounters with an
ogre ambush, the fact that the wizard’s ears are still ringing from the
thunderwave spell he cast at the
beholder, the overall noise level of the dungeon — none of that matters any more. They all cancel out.
So if you think someone in combat should have disadvantage on their check because much like the wizard they are not paying attention, then give them advantage.
It's not a house rule, it's part of being a DM and deciding when you think advantage and disadvantage should be applied for things not called out in the rules.