Ah ha! Thanks for pointing that out. It is a fact that sometimes the notes really do say 'and don't tell the heroes this...' and it's something (even written by Mr. Baker himself) that you could only discover by accident. I guess if it's a bonus (but not necessary) treasure, it's going to be just one of those things you never mention. How would it come up? "Wow that quest had nothing in it" "Well actually there was something..." "What, how could we miss it? We searched everywhere!" "Well, not everywhere..." "Yes, we literally searched every single room" "Well, but you didn't walk right up next to something to search now did you..." "When has that been a thing??" "Got 'em!"

???
How I feel about it is this... if you're going to just randomly change the rules like that, well, for some groups it won't matter in the slightest because the players are already doing that sort of thing unprompted (despite it not being required)... or else you're going to have people missing stuff if Zargon is thinking this is a practical puzzle that he isn't even going to hint at... otherwise he's going to have to bait the heroes with the suggestion, which may result in the heroes taking up lots of unnecessary time trying to maximize the searches every time there's furniture.
I think until this point the idea that there's furniture in the room can be a clue "maybe you should search this one, might be something special" especially if it's a certain piece of furniture. The first weapons rack was just full of ruined equipment, but it was kind of a clue hey maybe there will be something here. Treasure chests can be "empty" and so on.
I'm all for Zargon keeping people guessing, but requiring you be adjacent to the furniture to search to me seems completely a waste of time, though I could see the value in an adjacent search that requires an action but could be done when enemies are present, being valuable (and it's a risk not just because you're foregoing your attack this turn, but also because it could have some bad effect).
On the other hand walking up to furniture and searching each piece individually seems like the sort of thing you might do in a first quest when the players are still learning. But you'd have to at some point let them know it's not necessary. Is there going to be this same level of detail in micro-managing their actions in future quests? The FOMO will get them once you use it, I fear. Unless Zargon is like "Just so you know, in this particular campaign, sometimes you may want to be right next to something to do a special search of it..."
This to me is like one of those video game easter eggs. You can't ever make it something required or super important, unless you drop some clue or hint beforehand, or it doesn't feel fair. Then again, we've had stuff in HeroQuest before where it just kind of comes out of nowhere. The quest writer writes what he writes (and Zargon has total power over the game).
Thinking past my knee-jerk reaction, I guess the initial FL quest with the bodies does kind of IMPLY that this could be a valid tactic in the future. If you can search a corpse, maybe you can search other objects on the board too?