If the cleric is casting a spell to test whether or not it is a mimic, then I think the cleric has noticed a threat, by definition.The OP describes the room as 30' by 30' with the mimic in its center. I take that to mean the party is only 15' away which is just one move for the mimic, so it can move in and attack a party member on its first turn in combat. The cleric starts combat by declaring they target the "chest", and the entire party is surprised because no one has noticed a threat. I'd say the cunning hunter would know it's time to spring the trap.
As a player, if my character is sufficiently aware of a specific threat to be taking specific precautions, I would not consider it to be a fair use of the surprise mechanics if the DM ruled I was nevertheless surprised by the same threat I taking specific precautions against.