In the example, the DM describes that the party notices a troll. Then the players declare an action to attack the troll. The DM then decides the troll will fight back, so the combat rules are used, and the DM first determines if anyone is surprised.
Let's unpack that.
1. The party notices a peaceful troll. It is not a threat since it is not likely to cause harm to the party.
2. The party declares an action to attack the troll, forcing it to become a threat in that moment that combat begins.
3. The DM has to determine surprise. Per the rules, the party has to have already noticed a threat in order to not be surprised surprised by it. There was no threat to notice prior to the beginning of combat, so the party is surprised by the now threatening troll.
This is what happens if we follow your set-up and only use threat as the criteria for surprise. In order for the surprise rules to function, threat has to include potential threats like the peaceful troll above.
Okay, serious question then. Assuming this is how you actually play, why wouldn't everyone who plays with you always state their characters are expecting some specific thing in the environment to prove dangerous in some specific way? Is there some cost involved?
Because it doesn't work. I run my games so that a threat you don't notice can surprise you, like RAW states. Even if you are expecting the wall to attack you, the cloaker on the ceiling that you didn't notice will surprise you.
In addition to RAW, I also allow a threat you don't notice when combat begins to surprise you, even if there are other threats that you do notice.
Seriously? You're taking issue with the difference between something and a specific thing?
How could I not take an issue with someone conflating everything with one specific thing? They are vastly different things.
There's nothing in the rules that says expecting danger from a specific thing makes you impossible to surprise. Nothing.
Yes there is. It's called being aware of the threat or potential threat.
What isn't in the rules is the requirement you are adding to know exactly what the threat is. You have yet to show that you have to know that the chest is a mimic in order to not be surprised by your awareness of the potential threat that a chest poses.
I'll let the statements of the game designers in the core rulebooks inform me of the realities of the game, thank you.
That's silly. They get things wrong all the time. It's indisputable that a mimic that is unnoticed and gains surprise is a greater challenge than one you know about well before you get to it due to a successful perception check. You should look at reality yourself and make your own determinations of what reality is, not rely on game designers to tell you what it is.