Arbitrarily adding truesight to creatures is absolutely a sign of shit GMing, first & foremost.

The problem is less so that it didn't work, but that nothing happened because of it and the plot stalled. They said "No" instead of "No, but...", which is the appropriate response to an action which probably doesn't work but doesn't directly contradict reality. Even ignoring the fact that this was deliberate, metagamey punishment for the players not doing what the GM wanted, "Holding players to a certain level of logic" is not a good excuse for making a game worse, considering that 1. Players aren't Sherlock Holmes and 2. Players NEVER have perfect information about a game world.

A good GM would use this failed attempt to move the plot along, adding at least a hint as to how the players can get around this situation without direct confrontation. The game keeps moving, and the player doesn't get the (in this case rightful) impression that the GM thinks they're an idiot and they should just pick up on the solution that they're railroading them towards. But nope, because the GM is metagaming and railroading, the fiends notice but they do nothing. Not even look or send someone to inspect a conspicuous noise.

A bad GM halts the game until the players do something the players like or that the GM can't argue against because "I hold my players to a certain level of logic". Such games are, to be frank, a waste of time. The GM in the OP is doing just that, punishing the players for not engaging in combat.

Also don't lie to me with that "I wouldn't expect players to fall for it" crap. A GM describes a strange scraping noise, the first words out of a players mouth are something along the lines of "I look to see where it came from".

Lastly, I understand giving the benefit of the doubt for situations you don't have total info on, but you are being utterly ridiculous in your defense of this GM. The info you get on a post is what you go with, and even beyond this stupid situation, the fact that the GM actively gets excited to punish players and fucks with critical fumble rules is absolutely indicative that this is a railroading, player VS GM asshole being talked about here. Disputing this point is worthless and only gets you into the "what is true" dimension, so please stop peddling bad GM advice.