iserith
Magic Wordsmith
No, that would be the opposite of fair since you're skewing the benefit in favor of one group over another (PCs vs everything else), and not treating all inhabitants the same---ergo, unfairly. Threats, along with everything else, should be telegraphed as they would normally be, and detected with a success based on the PC's abilities. That's it. Anything else is coddling the players.
I disagree. It looks like you are confusing what it means to be fair and what it means to be impartial. Sometimes these things can be used interchangeably, so it's an easy mistake to make.
You're obligated to create a fair game for your players - friends and relatives sitting at your table with the goal of having fun. You are not obligated to be fair to monsters and NPCs that do not exist. You are obligated to apply the rules impartially so long as you're achieving the goals of play. But I'm not talking about applying rules. I'm talking about presenting situations in which the players have a fair chance of making meaningful decisions to turn things in their favor. Sometimes they will. Sometimes they won't. This isn't about "coddling" at all.
You seem to be applying the rules impartially. But was the situation you created fair? By my standards, it is not. It relied upon the players reading your metagame tells to determine whether they should slow up their pace or to actively state they were looking around when this is something that is already assumed under passive Perception (passive checks representing the average result of tasks performed repeatedly). Now, you and yours may consider this a fair and standard practice. I'm saying that I do not, for whatever that's worth, and some other people agree with me. There is no right or wrong in a matter of preference.