No, that isn't at all what happens if you run surprise like I do. I've already told you what happens many times, exhaustively and in detail, so at this point I can only assume you're arguing in bad faith.
And no, the surprise rules function just fine without defining threat that way. It's not a term of game jargon and, in the context of the surprise rules, should be understood to refer to a creature that participates in combat on the opposite side from the creature for whom surprise is being determined.
What I'm saying is what happens if you combine what you're saying with RAW. I'm not saying how you do it. No bad faith anywhere and I'd appreciate it if you'd stop implying it.
This implies the wall is not a threat even though the player expects it to be one?
I'm saying that I don't run things as RAW states, because the way I do it makes more sense. If you're focused on the wall and the wall is a mimic, you won't be surprised by it. If you're focused on the wall and the real threat is the ceiling cloaker, you will be surprised by it.
Okay, so you're fixing the problem caused by your "potential threat" houserule by having another houserule.
No. I'm fixing the issue caused by shoddy PHB language by having a house rule.
You can't notice a threat if it isn't a threat, and no peaceful troll is ever a threat unless you provoke it, at which point it THEN becomes a threat. So how do you notice it before determining surprise? It's literally impossible unless threat = potential threat.
Something does not mean everything.
That's what I've been saying, but you've been conflating my statements about being alert to the potential danger of one thing, with being generally alert about nothing in particular, which means everything.
Yeah, this "potential threat rule" you've identified here. That's what's not in the rules.
It's the result of the shoddy PHB language. The surprise rules only work if threat = potential threat, because prior to combat there are many potential threats that are not yet threats, such as the troll walking peacefully by.
Awareness of the presence of a creature is sufficient.
Not by RAW. By RAW has to be a threat, which means immediately hostile. Just noticing a peaceful creature isn't noticing a threat.
That's true of any creature that manages to surprise the party. False Appearance just allows the mimic an additional circumstance where it can try to do that. This is backed up by the fact that the feature doesn't affect the mimic's CR.
It is true that anything that surprises you is more dangerous. The mimic, though, is built to be weak without that surprise. If you're 60 feet away and start attacking it, you have two rounds before it can even attack you back. It's slooooooooooow, which is fine because it's designed so that you have to be close and be surprised by it. The vast majority of those other creatures you mention don't have that flaw.