I expect the DM to provide some manner of clue that indicates the statue is something of note.
Why though? By definition, gargoyles are indistinguishable from a normal statue...unless/until they choose to move. Why would you believe the DM is under ANY obligation to "telegraph" anything with clues?
...in ANY situation, as far as I"m concerned, but we'll deal with the one actually in the thread at hand and not make up hypotheticals.
My estimation, based only on the context provided, was that the DM had a face-down card in his or her back pocket instead of on the table (gargoyles posing as statues). The player was playing procedurally - if statue, then detect magic. (I'm wondering if the warlock has Eldritch Sight and relies on it heavily.) When the procedure failed to turn the hidden face-down card over and led to combat, then the player objected on the basis that detect magic should detect gargoyles. But really, there is an objection behind the objection in my view: The DM didn't show the face-down card on the table in the first place and this lends itself to the perception of the encounter as being a "gotcha." I gave several examples of ways the DM might have done this in my initial post in this thread.
Short of additional context to the contrary and based on my years as a DM and player, this is what I see as the heart of the matter.
This is a good deal of assumption on the subjects of 1) what this table's expectations and playstyle are, and 2) that they somehow align to yours.
I do not assume any "right" [as a player] or obligation [as a DM], nor even really any desire either way, to show or be shown "face down cards" [whatever that's supposed to mean]. It is my job, as DM, to know things the players don't. It's my job as a player to figure out what I can with the information I am given...and react accordingly. The players are not entitled ANY kind of "clue", beyond what is in the DM's descriptions of the surroundings/situation (and, arguably, their knowledge of the world at large), if the situation doesn't call for it.
If there are clues that the party picks up and tries to use to their advantage. Great. If they try and fail! Great.If it's a situation/scenario where clue-leaving/finding is a meaningful part of the game -like a mystery-based plot arc/ongoing quest/etc..., then great. If there's nothing there for them to find/pick up, GREAT!
But it's D&D. "Gotchas" happen - and I am not even going to begin to argue over the fact that "a PC ends up in combat...maybe takes some damage first?" is hardly something I would consider a "gotcha"...or the fact that "gotcha" is really the gargoyles whole shtick.
At the risk of putting too fine a blunt on it: My attitude to a player who would complain to this is: "Things don't always work out the way you think they should. Welcome to D&D...and life. Get over it, handle the situation you find yourself in [or don't and die], and move on...presumably, knowing better/having learned for next time that a detect magic spell isn't going to reveal a gargoyle (if not followed up on to find it won't work on elementals of any kind).
That, of course, is just according to all of
my years as a DM and player which, I believe like many of us here, are comparable to yours.