Maxperson
Morkus from Orkus
How does the portion I bolded not apply to the party seeing the Mimic posing as a chest? Especially since the party is suspicious of it.I think you (and possibly @Maxperson) are reading "notice a threat" as "notice as a threat". The word as isn't there. You can notice a threat without knowing it's a threat. You just need to become aware of whatever the threat is. In this case, it sounds like both the players and the troll are aware of each other, so what you said is not according to me.
That bold portion is also what we have been saying. Notice a threat in 5e = notice a potential threat. You see it, and it has the potential to be a threat.