Lyxen
Great Old One
Why? I mean, it's fine if you do this, there aren't gamer police, but the argument for why it should warrant disadvantage is unclear. If a creature is paying extra attention, sure, disadvantage is a cool choice, but I question the assumption that creatures automatically have additional focus to bring to bear. A creature in a melee, for instance, already has quite a lot on their plate, and I don't see how they have extra focus to place on watching a specific hiding place. I mean, a competent ally is going to know your hiding tactics and can incorporate distracting attack patterns to draw attention away momentarily, which is all that's needed.
There's this strange, stark space where people seem to be applying the thinking for hiding -- one where all other distractions fall away and it's only this one interaction -- observer vs hider -- that's considered. To me, part of a proficiency in Stealth is an awareness of how to take advantage of momentary distraction, and just hiding in the same place doesn't, to me, justify disadvantage.
Then it's your call as a DM, and it's fine because it's your table.
At our tables we make different calls because the skill of the rogue is inherent in his bonus, but choosing an obvious place to hide makes it more difficult to avoid notice ot to be noticed when you pop out again.
And once more, it's troubling how you constantly do not want to hear that the devs themselves have put the bar of awareness beyond your immediate adversary extremely high, to the point where invisibles (and not the ones with your personal flaw) are tracked effortlessly across a battlefield.
Just watching specifically a certain area for a dangerous adversary that you know is there should be comparatively very easy.
But note that this does not detract from the rogue skill at all. He might be good enough to pull it off despite the additional difficulty, no worries if that happens and all more power to him.